BD Rebuilder is a program designed to compress Blu ray video using the X264 encoder. BD Rebuilder can reduce a blu ray video to fit a 25gb BD-R disk or compress to fit either single or double layer dvds. You can remove unwanted material like extras or foreign soundtracks, do a movie only backup, or compress the entire movie and have the menus intact.
BD Rebuilder is not a decryption program for blu ray. You must decrypt commercially made movies prior to using it. The output from BD Rebuilder is still in Blu ray video format, and requires a Blu ray player, or software on a computer designed for this type of video.
Edit: I thought I'd better update the description of the program, as new features have been added since this thread was started. BD Rebuilder's main function is still compression of blu ray, but it can also output as mkv or mp4 files now. There are many presets for mobile devices. And there is an option to convert blu ray video to dvd-video as well.
BD Rebuilder is still in beta, and is free to use. The latest version is 0.31.07 beta (February 10, 2010). You can download it here: http://forum.doom9.org/showthread.php?t=143716
Please note the auxiliary programs necessary for BD Rebuilder to work, including AviSynth, ffdshow and Matroska splitter. There are links to all these free programs at the download site.