Here are a few links that will hopefully be useful to any player of DnD. I will add to these as I evaluate them. These links are tested and functional as of 8 June 2003.
Wizards of the Coast DnD page - the current producers of the game. This is not just a company site, it is a living resource for all afficionados and a must-visit for you.
Communities and nexus sites devoted to DnD and/or its offspring the d20 roleplaying system. Go here to find more resources than you could ever use and to make contact with other DnD gamers.
Community 3e This is a good-sized site with many resources and downloads for DnD 3rd edition and the d20 system. It also has a lively community of message forums, where you will quite often find me.
EN World This site is focussed on industry news and product reviews and information. There is also a boards forum.
DnD Adventure This is a user-fed community site with many features of interest to DnD gamers. It has a US-based online store.
Mortality.net Has a radio station as well as the usual user-contribution stuff and gaming news. I hang out on these forums too.
Planet AD&D The usual downloads, community submissions and forums.
Third Edition Net Another site fed by user submissions, but community-evaluated rather than edited by site moderators, so you can rely on gettting your stuff up andyou con comment on the work of others.
Resources for open-source gaming:
The Open Gaming Foundation A page about the open gaming license that allows the world to contribute material for the d20 system.