Roger64 provided a great example previously using LinLibertine, but I didn't see anyone mention the Fell fonts he uses in his linked blog post. These fonts are also free under the Sil license and gorgeous for this use, particularly for period books.
A link directly to the fonts page:
One interesting bit, I was just playing around with the Fell fonts in an ePub. The OpenType fonts don't have a dedicated small-caps typeface, but the small-caps glyphs are included in several of the fonts. I wasn't able to get small-caps to display with the OpenType font faces, is there something special that can be done in the CSS to accommodate this, or do I have to just use the dedicated ttf typeface?