crowbarDISK is a dictionary attack tool for the encrypted disks on Macs. It supports the filevault2 disks too. It just calls the diskutil cs unlockVolume with the passwords, so it's not so fast. It's completely free.
What's new in crowbarDISK
Note: MacUpdate Desktop and Finder do not report a version number for this app; the dev states it is 1.0.0.