[seynb]

Inputting Unicode in X Windows is easy. Just type:

  setxkbmap -option compose:ralt

Now, right Alt (AltGr) is your compose key.

Then, for example:

  à is Compose + ` + a
  ö is Compose + " + o
  ß is Compose + s + s
  ç is Compose + , + c
  € is Compose + = + e

In Emacs, the easiest way to input Unicode directly is to set read-quoted-char-radix to 16 in your .emacs. Then use C-q (typically bound to quoted-insert) to input a glyph directly via hex code point.

  ∀ is C-q + 2200
  λ is C-q + 3bb
  ∃ is C-q + 2203
  若 is C-q + 82e5
  世 is C-q + 4e16

An easier way is to dial up a character map program like gucharmap and copy and paste.

[SwellJoe]

I've always wondered why it wasn't the default in Linux to have a compose key configured, and I used to have it set in the distant past.

I just found it's editable in the Gnome settings editor. Browse to Keyboard->Shortcuts->Typing. Then click the "Disabled" next to Compose, which will allow selection of the compose key. The directions at the bottom of the settings window are simply incorrect, or don't apply to this particular screen, I dunno. Regardless, it works well for most common characters (the only language I regularly type with need for them is Spanish, so it covers all the bases there).