http://craftytemplar.wordpress.com/ -Blog by a friend of mine about roleplaying. Great place to find links to inexpensive or free resources ranging from out of date editions to papercraft.
The 3.5 SRD: http://www.d20srd.org/ -Pretty much standard fare for any 3.5 player.
http://www.giantitp.com/ -The OOTS comic, an excellent selection of gaming articles and content, and an in progress campaign setting. Also, a forum which is positively filled with people who can answer nearly any D&D question in a heartbeat.
http://www.dicelog.com -For online games, a nifty way for everyone to see everyone elses dice rolls. Also, ensures nobody is getting "creative" with the rolls.
http://www.myth-weavers.com/ -Online character sheets for all editions. It's got other junk too, but the nifty character sheets are the best part.
http://www.nobleknight.com/ -Best selection and cheapest prices I've found yet for rare, oddball, out of print, etc roleplaying materials and sourcebooks. I've gotten a fair amount of $5 D&D books from here, and found stuff I've been unable to get anywhere else.
http://www.crystalkeep.com/d20/rules/DnD3.5Index-Races.pdf -Index of all D&D 3.5 races, their details, and it lists what book they can be found in. Very helpful. Wizard's site has similar lists for classes and feats.
http://rumkin.com/reference/dnd/diestats.php# -Statistical fun with dice. Not strictly D&D related, but definitely useful.
http://www.rpglibrary.org/utils/meddemog/ -Town/City generation utility. Helpful if you're trying to create realistic urban centers instead of the usual shantytown with naught but a tavern and a magic item shop.
http://community.wizards.com/go/thread/view/75882/19546874/Master_Player_Race_List_Version_20 -Another version of the race list. I've found one or two on each that the other doesn't have, but both are pretty complete, so really, use whichever you find easier on the eyes.
