Encyclpodia

No. Not a typo.

Encyclopodia is an Open Source software project that brings Wikipedia to the Apple iPod. Encyclopodia can be installed on iPod generations 1 through 4, plus on iPod Minis. The English version devours 1.7Gb+ of space, which may not be a lot if you've a 40Gb iPod stuffed to the gunwhales with the complete Billy Joel canon, but it's a hell of a lot to give up on a humble iPod mini.
Does it work? I don't know. I'd rather have 30 Bob Dylan albums, thank you.

http://encyclopodia.sourceforge.net/en/library.html

Source: Open Culture