These are just a few essential links to get started with.  I will be adding more links as I find them.

Southern Tier Dart League For those who do not know, the STDL operates in the Binghamton area, and provides a great deal of support for our local tournaments.
Crows Darts Excellent general website run by Tim Cronin.  The article "How the Pros Do It" is an excellent resource for learning your outs.
Planet Darts This is the official website for the PDC, the only true professional dart organization.  Virtually any tournament televised in the US will be run by the PDC, including the Las Vegas Desert Classic.  Some highlights of the site are a series of articles on practice by John Part, and individual match results for the major PDC tournaments.
The Dart Thrower Operated by Karlheinz Zöchling, this site has a wealth of information on both the mechanics of throwing a dart and the mental game.  The "Third Slump" article may ring a bell with a lot of people.
Toronto Darts The largest darts-only website in Canada.