AgileTest from Polygenix (http://www.polygenix.com/ BrokenLink WaybackMachine disabled by robots.txt) is a commercial tool that generates JavaLanguage Imposters (dynamically generated MockObjects) at the bytecode level. ImposterObject can mock any non-final interface or class. AgileTest also generates Interlopers. InterloperObject enable dynamic object substitution at any point in the class under test, allowing for comprehensive mock-based testing of non-parameterized object collaborations.
Polygenix AgileTest is (or was when it was available) free for non-commercial use.