The Raspberry Pi is a credit card-sized single-board computer developed with the intention of promoting the teaching of basic computer science in schools. It is a capable little computer which can be used in electronics projects, and for many of the things that your desktop PC does, like spreadsheets, word-processing and games. It also plays high-definition video.
Libraries have used them for digital signage, programming with the public, OPACs, etc.
Find the manual here.
Buy here for $100