WriteBarrier provides a way to watch for changes to the state of individual objects. It's intended for use with persistence, transactional, or indexing mechanisms where you're managing a discrete set of objects of varied classes, and want to be notified when any of them are modified. I expect to modify my GOODS client to make use of this to speed up commits; it might also be useful for Magma, OmniBase, GLORP, etc.
It's quite fast and reasonably transparent, and has enough tests that I'm pretty confident in it, but it still does nasty voodoo stuff and so please test it before you load it into an image you care about.
It requires Squeak 3.7 (and not 3.6 or 3.8, because neither includes the BytecodeGenerator it uses).
For more details, see the test cases and the WriteBarrier class comment.