The more I thought about it, there are lots of great birding websites, much more than a Top 5 or Top 20 could really do justice too. Here are a few that should be checked out.
Birdlife International – A great organization and one I have not spent enough time exploring fully. Great information about the status and conservation efforts going on about birds all over the world.
American Bird Conservancy – My favorite thing about this site is the Important Bird Areas (IBA) section and their efforts to persuade people to keep their cats indoors!
Fatbirder – a wealth of information and a good place to find info about places outside the U.S.
There are many more great websites out there. Feel free to add a comment of your favorite ones!