TwitterKeys — “TwitterKeys is a browser bookmarket that pulls up expressive characters for easy copy & paste of all those crazy unicode symbols that people love to include in their emails and blog posts.” Very handy. See also: ⌘C ⌘V Character.
I prefer the one at http://panmental.de/symbols/
Does the same thing but a bit more sophisticated, with auto-select etc.
Oh, very nice. I had not seen that one.