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.

Great breakdown of digital signage programs

Manual here

Buy here for $100