My favorite character sheet, autocalculates in Excel, saveable
Shado - character sheet, power sheet, ritual sheet
WotC - official Ask the Sage for frequently asked questions about confusing rules
Four Ugly Monsters - community of gamers and resources
The Harrow - many excellent articles on gaming and the life of a gamer
Mad Brew Labs - online resources
ENWorld Forum - hundreds of files and resources on anything D&D
ENWorld - links to game aids
Heroforge and similar files for 4e
DungeonCrawl - power cards
Kiznit - 4e combat crib sheet all in one
Kiznit - 4e character sheet folder with card holder
Ander00's power cards (direct download)
Tools for DMs google group - online resources
Asmor.com - random encounter generator
D&D Adventures - a site filled with resources and adventures