Here are some tools that I have found to be indispensable for fine modeling, and others that add greatly to one's quality and productivity of work. 

Plastic, wood and soft metal tools:

Fine tipped jewelers tweezers - Like 'RR', 'MM', or 'AA'. Scale your hands down to HO.

Stainless steel razor saw blade - Just the blade, hand held. Much better control and can cut flush, but hard to find. 

Superglue applicator - a blunted dissecting needle works great to accurately apply ACC from little puddles on a glass slide.

Large sanding block -  a flat sanding surface of at least 9" x 3", and easy to replace sandpaper. I find that 150 grit aluminum oxide is fine for most general work.

Small, clear plastic boxes - to organize and protect small parts during construction and disassembly.

 

Metalworking tools:

Machinist/ Drill Press vise - 1 1/2" - 2" jaws, square on 4 sides so it can be used to hold things in various useful orientations. Also can be used as an angle block, and even as a weight. Micro-mark has a nice one for $25 or less.

Dremel tool with mandrel and cutoff disks - makes cutting and trimming brass very short work. Get a bulk pack of disks because they break easily and wear down (if you are very careful and don't break them).

Fine cut '000' 4" needle files  -  cuts almost as fast as coarser files. but leaves a smoother finish and can get into tighter spaces

Tapered reamers -  the Micro-Mark and Mascot sets are nice with knurled handles, but with a somewhat limited range. So have some of the larger types too for larger holes

 

Common Tools:

Stainless steel Dial caliper 4" or 6" - much easier then a micrometer for measuring drills, wire and sheet thickness. Can measure in many different ways, and desired dimensions can be calculated easily. Also can be locked at any point and used to scribe a line parallel to an edge as well as height/depth and inside measurements. I like Mitotoyo, but there are many good brands, just don't get the plastic calipers, they are not as accurate and you can't scribe with them.

Small 6" flexible steel rule - small and light for easy bench top layout, measuring, straightedge, and alignment uses.

Comfortable Xacto knives - keep a new sharp blade in one for precision work, another for general purpose, and another dull one for metal work. Different color handles help to quickly identify the sharpness of the blade. Then cycle the blades from sharp, to used, to dull. I advise getting a bulk pack of #11 blades so a sharp one is always at hand and they are cheaper in the long run.

Flush cutters - use one for quality, soft work, another for general purpose, and another for rough, hard materials. Some large, some small. Don't skimp on quality here.

Steel angle blocks and shim and dimension blocks -  Micro-Mark has one, or available cheap from used machinist stores. Used to set up accurate glue and solder joints, accurate sawing, layout, and as weights.

Screwdriver:  Wiha's are the best, if a bit pricey. There are cheaper knockoffs available.

Clamps:  You can never have enough different types and sizes !

 

Modeling Tool Sources:

Micro-Mark  'The small tool specialists'
North West Short Line Tools

Jewelers Supply:

Shor International
Gesswein
FDJ Tool

 

Home built or upgraded tools:

Oxygen/Propane Torch on the cheap

Tiny drill press

Dremel Table Saw upgrades and accessories