Librivox has a very large assortment of free audio books. The catch is they are all in public domain, so they are mostly older works and they are all recorded by volunteers. However, I can say that I've found them an extremely excellent resource (I'm listening through Hobbes' Leviathan now).
So, if you'd like some older thinkers or scientists (Kant, Hume, Locke, Newton, Faraday, Mills, etc), they're a great resource.
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My own recommendations are here. I haven't updated that list for a while, but I've been listing the books I've read each month over here instead, nearly all of which are audiobooks.
Blimey that's extensive, thanks a lot, I'll take a look.