Forgot my password to a disk image (.sparseimage)
CrowBar is somewhat out-of-date.

Probably the most comprehensive and up-to-date free password-recovery package is JohnTheRipper. Among many other formats, it can test .dmg's as well. 

Note that JtR does not "break" the encryption, it just automates and speeds the process of trying thousands (or millions, or trillions) of dictionary-based and random passwords in hopes of eventually hitting the correct one that decrypts your data.

JohnTheRipper is a command-line tool and requires one to be pretty well-versed in standard shell syntax, etc.

If you don't want to compile JtR yourself, there are also free unofficial pre-compiled Mac OS X binaries available.

The latest "jumbo" and "magnum" community-contributed versions can handle .dmg files, sparse-images through OS X 10.7. If you dig around in the recent patches you can even find code to handle OS X 10.8 passwords.