AltsFinder is an addon that helps to find other player's alts (alternate characters).
Feedback: if you have an issue or a feature suggestion or any other feedback - you can post it here in this thread (or use any other way suggested at the project's page).
How it works: The addon waits for the target to log off. Right after that, the addon scans the chosen zone for currently online players (while target chooses his next alt and loads into the world). After some fixed time interval (after target's alt presumably logs in), the addon scans for online players again (now they should include target's alt). Then two scan's (referred to as "before" and "after" scans) results are compared and any newly logged in players are added to the targets' stats database with a counter. Characters with a high counter are candidates to potential alts of the target.
There is an embedded help at the addon options screen (look for tabs' drop-down menus). You can also refer to the Wiki.