This addon by Tokipin allows you to make notes of characters.
Toki's TargetNotes is a simple target-based note-taking addon. Target a unit (player, mob, ...) and use the command /tn or /df, followed by the note. For example:
/tn good priest
Then whenever you see the target again, you'll see the note(s) you took for it. This basic mechanism is augmented by a search UI that lets you find notes and target info quickly. The search command is /ts. You can enter it by itself to bring up the window, or type a query with it to pre-search directly from the slash command. For example:
/ts gnome warrior
This will find all the gnome warriors you've taken notes on. You can also bind a key to toggle the search window in the Key Bindings interface.
Here's some potential uses for notes:
If someone gives you a good deal on something, take a note on them. Maybe they offer a certain enchant for a good price. Include the name of the enchant in the note. Next time you need it, you can search for the enchant to look up their name and message them.
Notes are a quick way to jot down someone's phone number/MSN/etc., as well as other things, such as how much gold you owe someone, or how hot you think they are on a scale of 1 to 10.
You can take notes on individual bosses to keep track of their idiosyncracies. Later this can help you remember the battle, and also make it easier to teach people who are new to the fight, since you will have a short list of the most important things.
Likewise for other mobs. Maybe there's a mob that casts a spell reflect. Whenever you encounter the mob again you'll see the note warning you to watch out for it.
Have you ever encountered an enemy player whose name is familiar, but you don't remember why? Maybe they were nice and actually helped you out a bit, or maybe they were that asshole that endlessly ganked your level 2 alt. You won't have the problem of forgetting those things if you take notes on notable enemies. The TargetNotes database is account-wide, so whenever you see the person again you'll see the note you took for them, regardless of what character you're on.