Wizards of the Coast – Dungeons and Dragons main site -This is the first stop to finding more information about D&D.

D&D Insider – A subscription-based online feature created by Wizards of the Coast.  It has a database containing all rule items (powers, feats, items, monsters, etc).  It also features an online subscription to Dragon and Dungeon magazines.  The Character Builder function is also quite popular, allowing you to print your own power cards for abilities as well as character sheets.

RPGA Main Site – This is the organized play section of Wizards of the Coast dealing with D&D.  They organize events in almost all gaming conventions.  It is a volunteer supported organization and there’s probably a local group near you that you can join (it’s free).  The main attraction of the RPGA is the living campaign system, where you are able to play and advance the same character regardless of where you go in the real world or how often you play.  It’s a great setup for people with busy schedules.  The current living campaign is set in the Forgotten Realms.

D&D Official Updates - A link to WotC’s main site where you can download the most recent updates to the various D&D books you own.  Big changes to things like Stealth, Skill Challenges and other things that might impact your game.

D&D Official Forums This is the official D&D forums.

RPGA Official Forums This is the official RPGA forums.

RPGA Yahoogroup Quite a lot of RPGA discussion happens over various yahoogroups.  This one is the largest regarding things of interest in the Living Forgotten Realms campaign.

LFR-Oxford This site has a description of all the current LFR scenarios (really, this site is a lot more useful than the official WotC one if you want to find out info regarding the mods) plus lots of other useful links if you play the campaign.

Warhorn Developed by a friend of mine (and I think the beta version was featured at one of my local cons), many conventions and game days organize their RPGA events using the Warhorn system.  It’s a great resource for finding games and is an amazing tool for convention organizers when it comes to mustering tables.

Optimization Thread This post on the WotC official forums has everything you ever wanted to know (and some things you never thought of) about optimizing your characters for 4e.

Try out D&D Test drive the 4th edition of D&D with the quick start rules provided by Wizards of the Coast.