This will be an ever evolving list of tools of the trade. Nothing is off limits. If it helps you, it could help other people.

For starters
Paint:
Black
White
Red
Cobra blue
Tan
Olive Green
Brown
Gray
Flesh Tone

Super Glue
Dremel or other rotary tool
Xacto knife

Occasional dabbler of customization
Super Glue Gel
Super glue accelerator or baking soda
Tape (masking, electrical,
Tamiya translucent paint (assorted colors)
Dremel or other rotary tool with assorted bits
Xacto knife with assorted blade types and sizes

Advanced Degree in Customology
Windex
70% or greater Isopropyl Alcohol
Nail buffers
File sticks
Dremel, assorted bits, assorted collet and nut sizes.

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Toothpicks - great for fine point detail painting, especially if you sharpen them to a finer point with the Xacto or by spinning them on a fine-grit sand paper

Sewing needles - I use them like I use the toothpicks, but they have a finer edge than I can put on a toothpick and are great for painting eyes

Mod Podge - Useful for making fabric effects (especially from tissue or paper towel) more permanent. (I used a thinned down soak of Mod Podge on paper towel to do the hoodie on my Mega Blok Rampart)

Stir sticks - cheap and better for mixing your paints than your paint brushes. Just buy the coffee stir sticks at your favorite box store.

I would add brown, gray and flesh tone to the list of colors. Also, a spray can of acrylic primer, a kit of mini screwdrivers (to take apart legs and such) and a pair of small wire cutters.

OB

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What about an actually Toolbox?

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Jeweler's saw. Invaluable for vehicle customizing. Can make more precise cuts than a dremel.

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Hot Glue Gun
Goo Gone
Paper Towels

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Wood burning tool with fine and pointed tips to sculpt fur, scars, and other details directly onto the plastic. It is also great to seal a pegged accessory to a strap or clothing article without the worry of the glue harming the plastic.

Tweezers, both hobby and craft(which are plastic coated and often with different tips).

Styrofoam block and Q-tips. I find this essential for priming a group of cast heads with spray primer.

Pin drills. Tiny hand twisted drills for making small holes in pieces where the Dremel may be too fast or cumbersome.

Twist ties and a hanger. You'll find this great for holding the primed body parts and odd shaped pieces.

Micro punches. There are inexpensive seasonal punches for $1 that often have in scale maple and oak leaves. Use these on a crumpled up paper bag for dry leaves, or crumpled up origami paper in chosen colors to do live leaf fall. The other types make great stencils for airbrush use.

Reveal bulb in a small clamp lamp. Believe it of not, often the colors we see or match can be different than what is seen due to the lights we paint under. Great for checking hues.

Pencil sharpener. You can thin an over-large neck ball by shaving it down on the right most edge of a pencil sharpener.

Micro scissors with a fine sharp tip. Great for trimming soft goods, hard clothing bits, and flash and sprue bits that made it past QA.

Silica gel packets. Save them in a jar or box, and place a few between your drying pieces. Especially for my fellow customizers stuck in humid climates.

Bottle of Acetone, and swabs, pads, and Q-tips. Great for removing factory paint or priming just the area to be painted on a piece where you are keeping the base plastic color.

Plastic sprue, both polystyrene and polyethylene. Somewhere along the ride you're going to need a neck peg extension, rifle plug for a backpack, or just a handle for a hammer. The needs are endless, and this is a universal accessory bit often thrown away.

Some sort of magnifying lends. It could be the Helping Hands, hobby lighted magnifier, or just a pair of inexpensive reading glasses in 1.5 or 2.0 degree of magnification. Invaluable for eyes, and other details.

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Oh, some more stuff for my list:

Foil or Metallic Polyester Tape (aluminum, copper, gold, etc.) - great for giving a bare metallic look to pieces, making mirrors (such as rear-views on cars)

Plastic Welder - You can get one that's like a wood burner or soldering iron, but I prefer one that uses heated air. It gives a more finished look and can give some really impressive joins.

Not really in my toolbox, but part of my toolkit: A small lazy Susan - Use clay to affix a piece to it, then you can turn it as needed, such as painting even lines.

And I'm surprised none of us so far has said Aves, Magic Sculpt, Sculpey, or any of the other sculpting materials.

I have many of the tools above in my chest, so I won't repeat them. Here are some I haven't seen yet.

Basic:
hammer and fine point chisels for cracking torsos (also a socket and a vice to safely hold the figure in place).

Channel Locks plus small socket for cracking 25A torsos.

Intermediary:
Xacto saw blades (large and small)

Regular Xacto blade refills (many - as I go through these a lot. pluv might also suggest a blade sharpener, but your mileage may vary).

Large paperclips to serve as pins when splicing parts together (cut down with wire cutters)

Vaseline as a release agent when using hot glue

Small paint jars with lids for mixing large quantities of paints

Different use of Mod Podge - thinned down, the matte version makes a great sealant

Rounded sanding sticks (from hobby shop). Help get to hard to reach areas.

Sanding sponges (probably same as some of the nail boards, but more pliable).

Advanced:
Waterslide decal paper and bonding spray

Micron Pens (assorted colors, mostly black) for detail work

Electrical tape (various colors). I use this to make holsters, belts, skirts, "sculpt" over problem areas of a figure, and emblems.

example of use as emblem (with printed paper as pattern):





More Advanced items from toolbox:

Magnets, assorted sizes.

Clamps, assorted (including some on long sticks from an airbrush kit, but which are great for spray painting too)

Safety goggles (every time I dremel. every. time.)

Box of kibble (neck balls and posts, leg screws, styrene scrap, rubber friction pads from thigh joints, etc... things I may need in a pinch to make something fit better)

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Also these. Don't know what they are called. But I call them all-snips. Basically a cross between wire cutters and scissors. Cuts through plastic with ease.

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Hair dryer.
Quicker and less messy that boiling water.

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bump: let's add to this list and then make a good wiki article for it!

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Modge Podge-Matte for sealing:
water down your modge podge a bit til it's almost like Milk, maybe a little thicker. Brush over the figure. As it starts to dry, a paper towel or qtip can help clear up excess amounts.

Toothpick:
Sharpen down the toothpick on one end to a point, the other end to a little thicker than the point (think almost a small square). Use the pointy end to do the whites of eyes, the "square end" to help with pupils.

Magic Marker-Assorted sizes
Want to make a joint black with no chipping? Lightly sand the area, then apply the magic marker over the jointed area. After doing so several times, and letting it dry each time inbetween, use a light coat of modge podge to seal it. Paint it the black color, the again seal it when it's dry. You should then have an almost paint chip joint in black.

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Something I thought I'd written about before, but a quick/lazy search didn't turn up anything: Bondtastic/FiberFix/LazerBond/various other brand UV resin pens. These are the "as seen on TV" pens you see advertising a glue that can mend things in seconds. You simply apply the 'glue', hit with the included light for 3-10 seconds (depends on brand), and you have a mended item. They don't really work so well as a glue, but the resin in them is the same resin used in some resin-based 3D printers. This means you can use them to sculpt using resin, giving a more plastic appearance than epoxies (plus, it's clear, so you can make clear pieces, as well).

It's what I used to turn the head from this figure:


Into this figure:


I also used it along with Blue Stuff molding compound to make a damaged Eel helmet for an underwater diorama:




(before painting it: )

I like this stuff because it fits my impulsive approach to customizing. I can use it to build up a work, and be sanding/carving/painting it in a matter of seconds, instead of having to wait for it to cure. Our local Dollar General carries them and will occasionally have them on sale for $5 (at the normal ~$10, they are a bit pricey) and I stock up on them then.


Also, speaking of Dollar General - they carry a line of cotton swabs that come in a round, lidded, plastic container. The swabs themselves come in handy for various customizing purposes, but the containers are just the right size to use for storing spare parts without them becoming too unmanageable. The containers are maybe 3" tall by 4" in diameter.

nice stuff in here. only things I can add are clothes pins for clamps or to hold parts on sticks while painting, and heat shrink. I use heatshrink on nearly every figure I build now.

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