the tools I use most frequently are combination wrenches (box on one end, open end on the other) and hex keys (both metric). those two are pretty indispensable. beyond that, it depends on what you think you'll work on.
other handy tools that come to mind:
- cable cutter/crimper
- cable stretcher
- pedal wrench
- spoke wrench
- cone wrenches
- crank extractor
- chain breaker
- cassette lock-ring tool and chain whip
there's really no end to useful bike tools, but odds are good that by the time you need a bottom bracket facing tool (for example), you won't need our advice about it.
a few companies
sell basic home mechanic kits that might be worth looking into. if not to actually buy, at least to get an idea what you're missing.
here's Park Tool's (I picked Park Tool just because they're well-known, not because I recommend them.)