WowAce

Gives you a place to keep track of information via to-do lists and notes. Checklists and notes can be set as global so that they show up across all characters on your account, or you can limit their visibility to the current realm, current character or current profile. Within each scope, you can create sub-categories to help you keep your notes and checklists organized.

Example:

• Global
  • Category1
    • note1
    • note10
    • note11
  • Category2
    • note3
• Current Realm
  • Category7
    • note13
    • note2
  • Category8
    • note4
    • note12
• Current Character
  • Category5
    • note5
  • Category6
    • note6
• Current Profile
  • Category3
    • note8
    • note9
  • Category4
    • note7

Possible uses:

• Shopping lists
• Gear wish lists
• Crafting notes or keeping track of who wanted what
• General to-do lists
• A place to dump mail message body text

Limitations:

• Note names are limited to 64 characters, the same as the mail system.
• Note bodies are limited to 500 characters (same as the mail system).
• Since drop-down menus can't have scroll bars, you'll want to limit yourself to less then a dozen or so categories in each scope.

Future plans:

• Quick copy of a note to the new mail message window?
• Output the body of a note to chat or whisper?

Alternatives:

If you want notes attached to map coordinates, try HandyNotes (not written by me). If you want notes attached to bosses or instances, try BossNotes (also not written by me). There also used to be an addon named "Torta" which offered similar functionality, but there's been no activity on that project since Mar 2009.