My absolute favorite at the moemnt is Gentium.
http://en.wikipedia.org/wiki/Gentium
Freely downloadable.
There are two versions (all available in Regular, Bold, Italics and Bold italics) - Gentium Basics and Gentium Book Basics.
Also Droid that was developed specifically for use on small screens is great. Google paid for developement, released the fong under Apache license.
http://en.wikipedia.org/wiki/Droid_%28font%29
Also have a look at
http://en.wikipedia.org/wiki/Liberation_Sans
http://en.wikipedia.org/wiki/Linux_Libertine
http://en.wikipedia.org/wiki/DejaVu_fonts
many people also like Fontin
http://www.josbuivenga.demon.nl/fontin.html
another noteworthy font is Lido STF
Lido SFT was designed specifically for Czech newspaper
Lidové Noviny, so it can survive relatively adverse conditions, and can be used to squeeze lots of readable text into small space.
I personally do not like the font, because on my reader the capital letters stand out on the page - they look much darker than the rest of the text. But the font itself is very nice, professionally done.
http://www.stormtype.com/free.html
All the above fonts are freely available. Some are truly free, some are for your personal use.
You can also try Microsoft core fonts.
If you install power point viewer 2007 you will also get all core fonts from Windows Vista.