SourceForge.net, www.sourceforge.net, claims to be the world's largest repository of open-source applications (and there's good cause to believe that). Thousands of different technologies are developed through and hosted by SourceForge.net.
Developer Shed, www.devshed.com, is a general development site that includes articles discussing the various programming languages and technologies you are likely to use.
WebMonkey, www.webmonkey.com, is very similar to Developer Shed, although it is broader in scope.
If you are intrigued by the open-source community and projects such as MySQL, see www.opensource.org.