The libtheora reference implementation provides the standard encoder and decoder under a BSD license.
The libtheora implementation depends on the following libraries...
You may also be interesting in testing our unstable releases toward libtheora 1.2.
Cortado is a Java applet to enable cross-browser playback of Theora content. The Cortado homepage contains useful information and download links.
The Wiki contains a list of Theora encoders and Theora players.