This is a dump for handy d20 links.
Player Tools:
The d20 System Reference
http://www.d20srd.org/
Link to the index of DnD Articles at wizards.com
http://www.wizards.com/default.asp?x=dnd/features
Official Errata
http://www.wizards.com/default.asp?x=dnd/er/20040125a
Grid Paper Generator
http://www.incompetech.com/beta/plainGraphPaper/
DM Tools
Fantasy Name Generator:
http://www.rinkworks.com/namegen/
Encounter Calculator
http://www.d20srd.org/encounterCalculator.htm
Random Dungeon Generator
http://www.aarg.net/~minam/dungeon.cgi
Fun Stuff
Giant in the Playground Comics:
http://www.giantitp.com/
Nodwick:
http://nodwick.humor.gamespy.com/gamespyarchive/index.php
Dork Tower:
http://archive.gamespy.com/comics/dorktower/
Link to the index of DnD Articles at wizards.com
http://www.wizards.com/default.asp?x=dnd/features
Link to the Official Errata for all published Wizards' D&D books:
http://www.wizards.com/default.asp?x=dnd/er/20040125a
More DM Tools
Free maps
http://www.dragonsfoot.org/maps/index.shtml
More Player Tools
Point-Buy Calculator that you can even link to the results of
http://yellowrex.com/tools/pbcalc.php
More Player Tools
Fillable PDF Standard 3.5 Character Sheet
http://www.nyrond.org/download/fillable_35.pdf
More Player Tools
*Huge* listing of feats, where they're from, and their details:
http://realmshelps.dandello.net/datafind/feats.shtml