I'll spare you my huge list of games. ;) Here is the top 15 freeware titles:

1) Starsiege: Tribes (A fantastic shooter, which still to the day is a fun, playable online fragfest. Grab some bots and go offline, too)

2) Ground Control (A squad based RTS that looks nice and plays like a dream, not too complex but with the right amount of depth)

3) Freespace 2 (Techincally not freeware, but fun all the same, a fantastic space sim with great combat and big, big ships)

4) Nexuiz or OpenArena or Alien Arena 2007 (All deathmatch shooters, and all super fun)

5) Return to Castle Wolfenstien: Enemy Territory (An objective based shooter with amazing production values, grab bots)

6) Anacreon (A 4X strategy game without all the fills, and under a megabyte)

7) The Ur-Quan Masters (A huge space opera shooter RPG action whatchamacallit)

8 ) Tremelous (An RTS FPS hybrid, strictly online)

9) Nethack (A hard but still classic rouge RPG)

10) Conquer Online (Diablo plus MMO plus Asian themes)

11) Cube (A Quake-like shooter with great edit features and massive comunity support)

12) Assault Cube (Take above sentence, replace "Quake" with "Counter-Strike")

13) System Shock 2 (Abandonware, but fun and scary RPG all the same)

14) N (A simple but hard physics based Flash platformer)

15) Warzone 2100 (A RTS like StarCraft, but you create your own units)

For good free FPS games:

OpenArena (New release on the 7th)
Starsiege Tribes
Alien Arena 2007
America's Army
FEAR Combat
(If you're really desperate) War Rock

I personally play Anarchy Online, which is a fun, free MMO in a sci fi setting. It's deep and will take some getting used to, but it's a WHOLE lot of fun, and without any payment there's no issue.

There are a whole lot of free MMO games out there, most of them being pretty similar to each other and WoW. Just go and search the free MMO sites on the net, and find one for your tastes.

I'll go by genre as well. It's listed in decending order, first being my best.

FPS: Tribes, Unreal Tournament 2004 Half-Life, Marathon 2

--Tribes was distinctly awesome multiplayer team based fun, and I still play it weekly. UT is lower because I have only played the demo, but I'm already hooked. Half-Life is so neat that there are things that other FPS' haven't been able to duplicate yet. Marathon was so ahead of Doom that is wasn't even funny, and its still playable today.

RTS: StarCraft, Seven Kingdoms II, Battlezone, Total Annihilation

--StarCraft is the ultimate strategic game. Seven Kingdoms mixes it up fairly well and makes a fun game. Battlezone lets you be in the action at FPS level. TA is on there because I own it, but never really played it.

RPG: System Shock 2, Morrowind, KOTOR, Star Control II, Deux Ex, Fallout

--System Shock 2 was not only the scariest game ever, but it was a great custom mix of RPG and FPS. Morrowind was a great open ended RPG filled to the brim with goodies. KOTOR was a fun Star Wars game, and that's enough to get on the list. Star Control II was open ended RPG in SPAAACE. Deus Ex was another customizable mix of FPS and RPG...and Fallout was just cool isometric turn based goodness.

Other notable games include Freespace 2, Sacrifice, and those damn Civ games.

--Freespace 2 still to this day is my favorite flight sim/combat. Sacrfice is a great, distinct RPG-RTS hybrid. Civ is just Civ, you know.

But to pick a favorite? Huh. I'd probably go with Freespace 2. So much fun.