You'll be surprised at how far you can go with a set of metric allen wrenches and a couple of screwdrivers. Don't be afraid to throw your allen wrenches away when they wear enough so that you can see how far they fit into the bolt. Worn allen wrenches will round out the bolt head. Then you'll have to invest in a drill and an EZ-out and a new bolt.

The next most common repair is chain and cassette replacements. To do that you'll need a chain tool, cassette lockring tool that fits your bike, and a chain whip.

Hub overhauls are usually an annual project and take cone wrenches that fit your hubs (also an ordinary 17mm open end wrench). Front and rear hubs commonly take different sizes.

Bottom bracket replacement may require a crank puller but that's not always necessary. Cartridge bottom brackets found on most newer bikes require a special tool with splines to match your bottom bracket. If you don't have a cartridge bottom bracket, it's best to figure out exactly what you need because there are lots of different possibilities. If you're on a budget, this is a good place to hold off until you need them.

A spoke wrench is needed for adjusting wheels. They come in sizes too. Good ones only cost a little bit more than not-so-good ones and are a whole lot nicer to use.

When you buy a cable cutter I think that it pays big time to step up and buy a good one. A decent one might by itself cost as much as a whole budget tool kit. However, if your cable cutter frays the cable or leaves one little uncut wire, what's the point? Here's where to spend the money you saved by not buying tools that you won't need and putting off buying bottom bracket tools.

I think that's pretty much the basic kit. Of course, you'll never stop buying tools. There are a gazillion special bike tools and every single one has it's purpose. Some of those purposes just don't come up very often.