An addon that keeps track of who killed you in world PvP, by name and guild and puts that information in tooltip. So when you are rolling on your main, harvesting and come across a lowbie enemy, your tooltip would indicate if that person (or his guild) has ganked your alt sometime in the past.
Only one I can think of off the top of my head is VanasKoS, which I haven't used in a while so not sure of it's 2.4 compatibility. Also can't remember if it shares info across toons... but worth a shot. Cirk's Badapples comes to mind too but not sure how helpful that would be.
Retired prior to 3.2, before all challenge was removed.