Addon To Check To See If Someone Is Online

Any addon that alerts you if someone is online?

Friends list, guildwork

Jinx is correct, other than the feature in guildwork that tells you when another person using the guildwork addon who is on your guildwork friends list there aren't any addons that do what your looking for.

One reason is that it would probably require sending a request for that information repeatedly to the server which can look pretty suspicious from SE end, if they decided to care to look. Kinda like if you were just sitting there opening and closing your friend list like some crazed stalker.