Project Gutenburg is great for free classics. If you like sci fi and fantasy, try
the Baen Free Library. Baen publishing has gotten a whole bunch of their authors to publish some of their books for free. A lot of them are the first in series, to hook you and get you to buy the rest of them.
Mobipocket is owned by Amazon, and is what the Kindle format is based off of, but can be read by a reader on the PC. It's not really a bargain though, they are about twice as expensive on there as the Kindle version off Amazon.