BitLet is a simple Java implementation of the BitTorrent protocol.
It is the library that powers BitLet.org (a BitTorrent client that runs entirely in the browser plugin, as a Java applet).
You can build the project sources using Maven, and execute a sample client by calling:
java -cp target/wetorrent-1.0-SNAPSHOT.jar org.bitlet.wetorrent.Sample $1
Where $1 is a .torrent file you have on your filesystem.
You can review this annotated example for an overview on how to use the BitLet library.
BitLet is distributed under the Apache license. See src/main/resources/license.txt for the full license terms.