Add the compose key to your Xmodmap file with something like

keycode 134 = Multi_key

The compose key has it's own unicode symbol: ⎄, which I'll use in the following composition tables.

Accents

  • ⎄`a à
  • ⎄'a á
  • ⎄^a â
  • ⎄ca ǎ
  • ⎄ba ă
  • ⎄oa å
  • ⎄!a ạ
  • ⎄.a ȧ
  • ⎄"a ä
  • ⎄~a ã
  • ⎄_a ā
  • ⎄;a ą
  • ⎄,c ç

Weird letters

  • ⎄AE Æ
  • ⎄ae æ
  • ⎄ss ß
  • ⎄TH Þ
  • ⎄th þ

Currency

  • ⎄=C €
  • ⎄|c ¢
  • ⎄-L £

Typography

  • ⎄<< «
  • ⎄>> »
  • ⎄"< “
  • ⎄"> ”
  • ⎄'< ‘
  • ⎄'> ’
  • ⎄.. …
  • ⎄p! ¶
  • ⎄os §
  • ⎄?? ¿
  • ⎄!! ¡
  • ⎄--- — (em dash)
  • ⎄--. – (em dash)

Math

  • ⎄oo °
  • ⎄^0 ⁰
  • ⎄^1 ¹
  • ⎄_1 ₁
  • ⎄12 ½
  • ⎄+- ±
  • ⎄xx ×
  • ⎄:- ÷
  • ⎄/o ø
  • ⎄mu µ

Business

  • ⎄oc ©
  • ⎄or ®
  • ⎄tm ™

Emoticons

  • ⎄:) ☺
  • ⎄:( ☹

As you can imagine, the list goes on and on. The compositing system depends on your application (see the Ubuntu wiki for Gnome notes), but if you're using the X Input Method (XIM), your compose table depends on your locale:

$ echo $LANG
en_US.UTF-8
$ grep $LANG\$ /usr/share/X11/locale/compose.dir
en_US.UTF-8/Compose             en_US.UTF-8
en_US.UTF-8/Compose:            en_US.UTF-8
$ less /usr/share/X11/locale/en_US.UTF-8/Compose

If you want to customize your compose keys, just add your own rules to an ~/.XCompose file:

$ cat ~/.XCompose
include "%L"

<Multi_key> <h> <o> <m> <e> : "http://tremily.us/"  # Home page

Read Compose(5) for more information. You may need to log out and log back in (or use the newgrp trick) for your new ~/.XCompose file to take effect.