Sorry if this has been mentioned before, but I just stumbled (well not exactly, I was looking for information on pre-socratic philosophers and found a link on Wikipedia) on this site, which seems a very rich and serious ressource on philosophy.
It's free to read online, and for $10 a year you can download individual entries in pdf format, which might be useful for some very large entries. I'll have to download the sample pdf and see if I can read it on my Cybook.
Anyway, thought this could be useful to others...
http://plato.stanford.edu/
Florence