Now that SqueakMap's back up, I can post this:

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.