Ever tried typing the characters like: ™ or ® or ½? Can be done very easily in Ubuntu using the compose key. Check this link to find out how. A list of supported sequences on Linux can be found here.
Compose key covers a subset of the Unicode characters you can type. For more Unicode characters, check this link. List of some common Unicode symbols.
Update: To enable compose key in Ubuntu 12.04 (Precise), 12.10 (Quantal) and 13.04 (Raring) go to System Settings ▸ Keyboard ▸ Layout Settings (at bottom left) ▸ Options ▸ Compose key position. As I seldom use the menu key, I am using it as my compose key now.