Valve recently updated Steam and released a version for Mac OS X. What’s even better is that they treat a game license as OS independent. If you own Portal, you own Portal regardless of which platform you play it on, PC or Mac. While Mac OS isn’t my primary OS, it’s still nice to know I can kick back and play the same awesome games on that as well.
On a side note, my Steam library is up to 216 entries. Now, some of that includes things like the single player and multiplayer versions of Call of Duty: Modern Warfare 2, but hey. Steam is quite evil on the pocketbook. 
Tags: Mac OS, PC, steam, Valve Software