These are cool programs that I deem neccessary for you try:

Games:
Garry's Mod (Mod for Half-Life 2, free for version 9 and below, $10 for version 10)
Soldat (2d side-scrolling multiplayer military game, shareware, $9)
Clonk Planet (2d side-scrolling Age of Empires-like game, free)
Notrium (2d addictive top-down survival game, free)
N (2d addictive Lode Runner-like game, free)
Pocket Tanks (2d tank turn-based strategy game, shareware, $16)
Bontago (3d multiplayer block-building game, free)
Orbiter (3d spacecraft simulation game, free)
Gunz (3d 3rd-person multiplayer role-playing game, free)
Apophysis (Flame fractal rendering program, free)
Celestia (3d universe simulator, free)
Wazzal (2d top-down space shooter, free)
OpenRPG (Multiplayer mostly text-based RPG system, free)
Truck Dismount (3d ragdoll physics simulator, free)

Utilities and Stuff:
OpenOffice (Alternative to Microsoft Office, free)
Paint.NET (Alternative to PhotoShop, free)
Audacity (Audio recording and editing program, free)
Terragen (Photorealistic 3d landscape renderer, free)
ArtRage (Really realistic painting program, free)
Google Earth (3d satillite image viewer, free basic version, $20 "plus" version, $400 "pro" version, "enterprise" version too expensive to post price on the web)
Skype (VOIP (Voice Over Internet Protocol) client, free)
Firefox (Web browser, free)
Chrome (Web browser, free)
GomPlayer (Audio player, free)
Trillian (IM program that supports AIM, ICQ, IRC, YIM, and MSN, free basic version, $25 "pro" version)