I don't know if such a program exists but, I have an idea and I wonder whether this is available in C or not.
Think a program and a forum. There are members on forums. You enter nickname of one of, or more, members to the program. It starts checking the boards for that nickname if he/she is online or not and alerts when that member goes online until you close the program.
I imagine a similar one with Windows Live Messenger. You enter your friend's Hotmail account and it checks for them. There are add-ons doing this job, but I have an additional idea. There are free SMS services around the web. The program is going to check for people's online status and alert you via that free SMS services even you are away from your computer. It may sound an utopian scheme but, impossible is nothing. :)