Some assistance by Spiggy, of CalgunsSee also: Firearms Measurement Tools
The list below is separated into a number of categories. Overall, it represents a more or less logical progression from basic gun owner tools, to small-business level requirements. I'm not particularly interested in documenting consumables here, although there are a few overlaps. Generally speaking, most of the categories will assume you've already purchased the items in the lower-tiered categories.
A bore snake is useful for rapid cleaning, although the rod is more traditional and versatile. You can live without a bore snake, but not without a cleaning rod.
Toothbrush - optional but useful.
Light Gunsmithing Tools
These tools will end up in the arsenal of nearly everyone who does things with their weapon beyond the simplest of cleaning.
Bore lights allow you to inspect the condition of guns' barrels.
Boresighter - laser or optical; useful for rough calibration.
Clear work area (or magnetic bowl) - to avoid losing small parts
Dental mirror (optional but very useful for inspecting chambers and throats)
Digital camera or camcorder (optional) - to have a record of where parts were, before you came along.
Dremel or Roto-Zip
Files
Flashlight
Hammer
Needle nose pliers
Magnet (optional but very useful for picking up ferrous parts)
Magnifying glass
Left-handed drill bits - useful when dealing with stripped-out screws. It'll loosen the screw while you're drilling into it, then you use a left-handed tap to remove the screw.
Levels - for mounting scope rails or fitting scopes.
Picks (very useful to reposition wires and springs)
Screwdrivers
Wrenches
Torque Wrench (for scope ring screws)
Vise
Refinishing Tools
Airbrushing kit (optional, for applying certain paints)
Black paint pen (good for covering up nicks on painted steel)
Paintbrushes
Sharpie marker (good for covering up nicks in parkerized steel)
Sander (or just use sandpaper / steel wool)
X-Acto Knife - for fitting stocks.
Building or Modification Tools
This does not include specialized tools, such as AR-15 armorer's wrenches or AK-47 rivet jigs.
Angle grinder or Roto-Zip
Barrel vise blocks - to hold barrels steady while installing things on them
Blowtorch - for spot heat treatment; unnecessary for most purposes not involving AK's
Bolt cutters - for shortening screws or bolts as appropriate
Drill press
Hammer, 3lb
Headspace gauges (go, no-go, and field, in the calibers being worked on)
Heat gun (for drying parts)
Heavy duty pliers (channel lok or visegrips ideal)
Sharpie or other marker (for marking cuts or covering scratches)
Shop press - where AK-47's are concerned, this is mandatory to progress beyond bottom-tier builds. 6-ton may work, 12-ton is the minimum effective power, 20-ton is ideal
Spot welder - recommended over normal welders in certain, very limited cases (i.e. rails on AK's)
Strong magnet - when building, scraps of steel get dropped and need to be picked up efficiently.
Taps and dies - may be required for some builds.
Wire clippers - for shortening springs
Welder - necessary for CETME or G3 builds.
Top-Tier Tools
When price is little object and the desire is to do the job as effectively as possible.
Cleaning patches. In addition to these specialized gun-related items, depending on what you're working on you may need a screwdriver with several different bit-sizes and types, including flat, hex, and philips head.