The OpenData StackExchange site has just launched in beta, and looks to be a great resource for open data sources. Like StackOverflow for programming and CrossValidated for statistics, OpenData is is a question and answer site for developers and researchers interested in open data. There's no R tag yet (though that would be nice for data sources specifically compatible with R), but there are already some useful tags for government data, APIs, and tools (to name just a few). If you have expertise on open data to share, or just have a question you need an answer to, follow the link below and check it out.
StackExchange: Open Data beta (via JM)