First off what will you be powering? One CD/DVD drive, one hard drive, a low power video card, etc you won't need much. Do you want it to be super quiet with no fan? Are you running a server with 15 hard drives and need redundant supplies as well?
The biggest thing is a powers supply that delivers consistent power, i.e. the +5-, +3-, +12-and power rails stay where they are supposed to. They will fluctuate a little, but you don't want them going all over the place.
Take a look at this article from Tom's:
Other hardware site have some power supply round ups just do a search on Google and you'll find them.
You don't need to go crazy with the power supply get what you need, but also prepare for future needs. Intel is pushing a new from factor called BTX so power supplies will be going through a change. If you have SATA drives you might want to look at PSUs that come with SATA connectors so you don't have to use those adapters.
Another cool PSU innovation is the modular connection. This allows you to only use the connections you need and remove the ones you don't. I wish my PSU was like that extra wires make a mess and can inhibit airflow in a case.
Frozen CPU also has a good selection of PSUs
I have an Enermax 350watt quiet PSU powering my system; 2 80 gig hard drives, 1 lite-on CD-RW, AMD proc overclocked, and an older GF4 4200. The newer ultra high end graphics cards from ATI and Nvidia are real power hogs so that is something to keep in mind. I like the fact that it has a fan speed controller so I can adjust the fan/noise. That might be a feature you don't really care about.
As with anything just do the research and pick the one that's best for you.