Current features include the ability to block HTTP requests from being sent, and the ability to insert/delete arbitrary headers from the HTTP request or response (including cookies and referrer info). These can be trigerred based on a fully-featured set of matching criteria, including IP address matching, regular expression match against the requested URL or hostname, and several others. Much more extensive functionality is planned for future releases.
The code is pretty rough at this point, almost to the point of embarrassment; javadoc is nonexistent. The current release (0.1.0) is primarily intended for developers, the curious, and other such psychotically warped people. Though I use Transept for daily browsing, this is definitely an early alpha release; many functions have received minimal testing, and the code hasn't been proven in long-term use.
I'll put something more comprehensive on this page eventually. Meanwhile, visit Transept's SourceForge project page, where you can Download the latest release.