For me, tools for building has become a second hobby in itself! My most recent aquisition was a set of permagrit tools.
Some of the tools I use regularly are:

xacto & lots of #11 blades plus an assortment of other blades
A variety of steel and clear flexible rules, triangles and squares
Great planes sanding bars of various lengths
Dremel with flex shaft and a large assortment of diamond, stone and carbon steel bits
I picked up a really nifty router bit set for cutting rabbits using the Dremel.
Bench belt/disk sander
Clamps clamps and more clamps
A set of hss drills
Mini files, diamond files, riffle sets and mini rasps
Covering tools, irons and heat gun
Great planes power plane
Razor plane

The sky is the limit. I also use a variety of other hand tools, power tools and air tools too.
Building is the part of the RC hobby I like most, particularly scale. I went all out and spent most of last year turning my garage into a workshop, complete with custom cabinets. For a buildboard, I went with a 4' x8' island with a 16 guage cold rolled steel top and use magnets instead of pins. Like I said, tools are a second hobby in itself. If I could afford it, I'd get a cnc lathe, mill, and a laser cutter too!

Scott