My basic tools are a round and flat file, a jig saw with a fine metal cutting blade, a drill with bits ranging from about 1/8" to 1/2", sand paper, zip-ties, and electrical tape. Maybe some screw drivers, wire cutter, tin snips, and needle-nose pliers. The drill and jiz-saw are the only big ones and they can be had for about $20 for basic ones. Most of the other tools are $5 or less. Everything can be baught at a local hardwar or home improvement store easily.
It is also nice to have a few extra fans and heatsinks laying around. Some of the older Pentium Pro heatsinks are great for hacking up into smaller custome heatsinks for RAM sinks. You can find them cheap (used likely) at most local computer stores. I picked up a nice bundle of about 10 for $2 and have usede them on many of my cooling projects. Arctic Silver II (or better) and Super Glue is nice to for attaching the RAM sinks. Wire (and a soddering iron if you want) is also nice for lengthening (or shortening) wires to route them better. Scrap 12g aluminumj sheets are handy too for making brackets and stuff. Lighted magnifying glass is nice if you want to do very fine detailed work. Some fan grills and lots of screws and small bolts and nuts also helps.
Masking tape, old news papers, fine sand papers, rags, and some automotiveprimer and paint is also handy if you want to change the paint sceme on you case.
A nice anti-static mat or grounded wrist stap might also be nice if you assemble a lot of computers. Static electricity can be a bad thing for computers.