Let's do an as complete list as we can produce.

Mac <-> PC Networkable games

4x4 Evolution
4x4 Evolution 2
Aliens vs Predator 2
America's Army
Battlefield 1942
Battle for Wesnoth
Black Hawk Down
Call of Duty
Call of Duty: United Offensive
Call of Duty 2
Close Combat: First To Fight
Command & Conquer (The original in OS 8)
Deus Ex with multiplayer patch
Diablo
Diablo II (OS 9, OS X)
Dominion 2
Doom 3
DropTeam
Ghost Recon
Halo
Hearts of Iron 1 & 2
Heretic II
Hexen II
Jedi Academy
Jedi Knight II
Lineage
Lux
Marathon (Aleph One)
Master of Orion 3
Massive Assault
Medal of Honor
Myth II: Soulblighter
Myth III: The Wolf Age
Myth: The Fallen Lords
Neverwinter Nights
Nexuiz
No One Lives Forever
No One Lives Forever 2
Quake
Quake 2
Quake 3
Quake 4
Rainbow Six
Red Faction
Return to Castle Wolfenstein
Rogue Spear
Rune
Shadowbane
Sin
Shogo
Soldier Of Fortune II
Star Chamber: the Harbinger Saga
Star Trek Voyager: Elite Force
Star Trek: Elite Force II
StarCraft
Star Wars: Battlefront (but shaky, needs patch yet to be released)
Tony Hawk's Pro Skater 4
ThinkTanks
Tron 2.0
Unreal
Unreal Tournament
Unreal Tournament 2003
Unreal Tournament 2004
WarCraft
WarCraft II
WarCraft III
Wakeboarding Unleashed
Wolfenstein: Enemy Territory
World of Warcraft
WW2 Online
X-Plane
XIII (requires you to have the IP of a PC server as there is no in game list).

Expansion packs to all listed games usually work as well.

Mac <-> PC networkable demos

DropTeam
UT
UT2k3
UT2k4
Q3
most Q3 engine based MP demos

Ah, Counter-Strike isn't supported? No Half-Life? Damn. I was going to buy a Mac but I love these games.