GuildWho lets you find which alt character someone is online on or list someone's alternative characters. It uses the public or officer (configurable) note to deterrmine which characters are linked to the same player.

So how do you set up your notes so you can use this addon? All you have to do is make sure that every alt has its main character's name (important: no extra stuff, just the name) as its note, and the main character's note anything that contains any non-letter character. (be it an empty string, a name with surrounding brackets, digits etc)

Because of limitations in the API, if a note was updated in the last 10 seconds, your results may not be accurate. ;)

- Usage:
/gwho character - Find who this character is online as.
/alts character - List this character's alts.
/gwho !setnote - Toggle using public/officer note.
/gwho !details - Toggle displaying alt details.
/gwho !debug - Toggle debug messages.

You can also right click someone's name in chat or guild roster and choose to display that person's alts.