Twitlist

Description

This extension allows you to add personal custom notes to any Twitter user. The note you write about a user will appear on every one of their tweets in your feed. This is great if you ever forget why you followed someone, or vaguely remember them being a jerk but don't know why, or are about to retweet someone you forgot was an awful person (or a pathological liar).

To use it, install the extension. You'll only be able to see the icon in your extension bar when you are on the twitter.com site. You'll need to click on the icon and use the popup to log in with your Twitter account. In order to add notes to the list, click on the "add a note about this user" beneath the user's name in their Tweet. If the user's already been added, you'll see the note and a "view on twitlist" option that lets you edit it or manage your lists.

Both of those links will take you to the twitlist web app, which lets you manage your lists and share them with other people. Lists start out private by default, but you can share them with select users or with everybody.