I actually use Ubuntu 10.10 as my primary Operating System and I do play all of my Steam games on it using the PlayOnLinux project and WINE. While you do need to do a bit of console hax to get it working like you want it to, it's a top at just random web browsing and such.
B0red provided a good steam client for Ubuntu, no doubt but, if you also want to run your games, I would suggest looking up the PlayOnLinux project.