As far as I can tell the IM programs I use (and Skype) have the feature to notify you when people on your buddies lists come on or off the system. A feature I find increadibly annoying and turn off instantly. Way too many notices poping up on my screen.
However, it would be really nice to have the ability to set a flag on an individual on your list to let you know when they specifically came on our off the system (on a one-off basis). I do a lot of work with folks in different time zones and particularly on Skype it would be great to be able to say "tell me when THIS person next comes online." That would be really handy. Just right click on the person's name and set a next log-on notification.
Update: Figured out that AIM has this feature - Skype needs it.