Philosophy books are expensive. Never fear: Librivox has loads of free audiobooks. The search is not the most efficient I’ve seen, but there’s definitely some Kant and some Mill (and some HP Lovecraft) on there, so it’s hard to complain. (Though I’m kind of in agreement with Brian Leiter when he comments that “Of course, cruising down the highway listening to someone reading the First Critique could be a safety hazard…” On the other hand, any crashes would be mere phenomena, so perhaps nothing much to worry about after all.)
Another source of freebies is the James Rachels site, where you can download some of his work.
If anyone else comes across freebies of interest, do email me and I’ll publicise them here.