There are copies of The Feynman Lectures on Physics online at
http://dl.dropbox.com/u/41824698/Volume1.pdf
http://dl.dropbox.com/u/41824698/Volume2.pdf
http://dl.dropbox.com/u/41824698/Volume3.pdf
They're largish pdfs, so might take a while to download.
(I don't know who I'm stealing from by posting these online, but if you enjoy them then you could probably do worse than making a small donation to Caltech.)
EDIT: Now free and legal to read online here.
http://www.paulgraham.com/articles.html
http://lesswrong.com/r/discussion/lw/2ul/free_hard_sf_novels_short_stories/
http://www.raikoth.net/Stuff/story1.html
http://www.orionsarm.com/xcms.php?r=oa-story&story=dr_yes_jolonah (Warning! Scary!)
http://www.kuro5hin.org/prime-intellect/mopiidx.html
http://lesswrong.com/lw/3oa/i/
http://lesswrong.com/lw/y5/the_babyeating_aliens_18/
http://www.fanfiction.net/s/5782108/1/Harry_Potter_and_the_Methods_of_Rationality
Have to say I wasn't too impressed with "Yes, Jolonah". Its protagonist's predicament, while undoubtedly horrible on a conceptual level, didn't evoke much in the way of emotional horror -- the presentation was too melodramatic, the premise too fundamentally silly, and the characters too flat. The tvtropes word for this sort of thing is "narm", I believe.
On the other hand, I've always liked "Prime Intellect", despite its faults and despite the Luddism it feels compelled to devolve into.
While it's true that it's hardly a great piece of literature, and there are certainly more enjoyable things out there for the same time, it still IS enjoyable and I think it teaches an useful concept.
The raikoth.net story is awesome. "Yes, Jolonah" is not worth reading.
Edited ages and ages later: "Yes, Jolonah" is a frightening story that caused me distress due to empathizing with the characters. If you aren't prone to that or if you enjoy it, go ahead and read it. It is moving and well-written.
The cost of a book is negligible compared to the cost of the time it takes to read it, and the cost of filling part of your brain up with its contents. Choose what you want to read, and get it, whether it's free or not.
If you value your time at $20/hr, then unless you decide that you would read a book for the amount of time it takes -- but not an hour longer -- then most books are worth buying.
Random serious stuff possibly related to AI I've been looking at.
Judea Pearl: Causal Inference in Statistics: An Overview. Pearl's stuff seems to be pretty central to formal causality, and this is reasonably recent.
A Philosophical Treatise of Universal Induction. Solomonoff induction, related to AIXI stuff.
Shane Legg's PhD thesis Machine Super Intelligence
A New Approach to Quantum Logic, via Shane Legg, this is supposedly related to Bayesian inference and how humans do reasoning.
Physics, Topology, Logic and Computation: A Rosetta Stone A generally neat thing about category theory going into everything.
I'm going on holiday tomorrow. What freely downloadable reading do people recommend? I'm after both more serious things related to Less Wrong or SIAI/FHI's mission, or more lightweight stuff that is the sort of thing an LW participant like me would enjoy. Thanks!