The compose key may be used to type unicode characters easily on Linux systems…

All of these commands should be executed as a normal user, no root access required:

1. Identify the key you want to use as your compose key, I use the ‘Windows’ key:
   • Run the application xev
   • Press the key you want to use as your compose key
   • Note down the keycode value (my Windows key identifies as keycode=133)
2. Edit the file ~/.Xmodmap and insert the line
   keycode 133 = Multi_key
3. Run xmodmap ~/.Xmodmap

Restart X and you will be able to use unicode:
<Compose key> + " + u = ü
<Compose key> + ' + e = é
<Compose key> + o + r = ®
<Compose key> + - + y = ¥
and many more…

(Tested and in current use on Debian Wheezy)