Last week, Steam for Mac hit the ground running with a trunk-load of titles and deals. As promised, every Wednesday they will make more games available for Mac and this week they introduced a few cross-platform multiplayer titles.

You know what that means? Mac VS PC! You can log on and own your friends, no matter what computer platform they are running Steam on. The first three titles for the cross-platform endeavor are Killing Floor, Altitude and Madballs in Babo: Invasion.

John Gibson, the president of Tripwire Interactive and the company that developed Killing Floor is excited about Steam’s entry into the Mac world:

“We were thrilled when we learned that Steam was coming to the Mac platform and started working on a native Mac version of Killing Floor immediately. Steam’s advanced multiplayer support on both the PC and Mac has enabled us to bring Killing Floor to a whole new audience on the Mac and introduce them to the existing PC player community.”

The introduction of these titles on the Mac isn’t the stopping point, in typical Steam fashion you can get these titles at a sweet discounted price. For a limited time, Killing Floor is 33% off it’s regular price, and Madballs in Babo: Invasion is 50% off. You can play Altitude for free this weekend and pick it up for 50% off after the free playing time subsides.

Additionally, Steam unveiled more single player titles this week such as Sam and Max: The Devil’s Playhouse Episode 2 and Hearts of Iron 3. Even more titles will launch next week.

What games are you looking forward to play via Steam on your Mac?