Here is a list over companies and interest groups that have produced very enjoyable products.
Valve - The creators of Half Life, Portal and Left 4 Dead. All awsome games.
Forgotten Hope 2 for BattleField 2 is an awsome World War 2 Mod.
Point Of Existence is another great Mod for BattleField 2.
Official Blizzard homepage - The makers of World Of WarCraft and StarCraft 2.
DICE - The creators of the whole Battlefield franchise, which I enjoy playing alot.
MAME - Multi Arcade Machine Emulator. Where the old video games still live.
C64.Org - A homepage network dedicated to the good old Commodore 64.