The Stanford Encyclopedia of Philosophy definitely deserves a thread of its own.
This is one of the most impressive encyclopedias of contemporary Anglophone ('analytic') philosophy available anywhere. What's more, it's available free, online!
Here's its main page:
http://plato.stanford.edu
Here's its rather amazing table of contents:
http://plato.stanford.edu/contents.html
Each article was written by one of the leading authors in that particular subject. Many of the articles reflect the cutting edge of current scholarly discussion. The online format means that articles can be continuously updated as things evolve.
There are strengths and weaknesses. The encyclopedia is stronger in some areas of philosophy, such as the philosophy of science, than it is in others. But it's still growing. Many of the links in the table of contents don't work, as those articles have been assigned authors but aren't completed yet.
This thing is obviously intended as a continually-updated reference resource for university students and for professional philosophers. But reading widely in this could serve as a free non-credit, non-degree philosophy education for others who might be interested. A lot of it might be too advanced for beginners, but if you've had some introductory exposure to philosophy, then this thing could serve you very well.