WowAce

It's easy to add announcements for Purge and Spellsteal, similar to the interrupt announcements. Each could be disabled separately. (A spellsteal is really just a purge though; it would probably use the same settings as purge, and just optionally have a different announcement message.)

Tracking and warnings for certain debuffs, parallel to the tracking and warning of casts, would be good too, but is more work to add. I also have to figure out where to put the config for it without increasing clutter.