Important: Work on a library-based STM for Scala has shifted to the ScalaSTM project, whose home page is at http://nbronson.github.com/scala-stm. The source for the ScalaSTM project is at http://github.com/nbronson/scala-stm.
The ScalaSTM API draws from CCSTM, but includes design improvements by the Scala STM Expert Group and the ability to dynamically select the underlying STM implementation. ScalaSTM's reference implementation was started from the nesting branch of CCSTM, which adds partial rollback to the core CCSTM algorithm.
Documentation for CCSTM is available from http://ppl.stanford.edu/ccstm.