A few weeks ago, I switched from Windows to Linux, and one thing I'm missing is Altnnnn keyboard shortcuts to insert an em dash and other things. Is there any way to get them working under Linux? I'm using Arch Linux and KDE, if that matters.
Using numeric code points
In Tobu's answer, I believe the keystroke combination on US or UK keyboards would be called Ctrl+Shift as in "Ctrl-Shift-2702 is for the scissors character (✂)".
Personally I find it much easier to remember digraphs.
Digraphs / Compose-Key / Multi-Key
The X Window system (X11) provides for what is sometimes called a compose key which allows the entry of special characters by using digraphs.
Shift+AltGr ~ a --> ã (ã in HTML) Shift+AltGr / o --> ø (ø in HTML) Shift+AltGr o c --> © (© in HTML) Shift+AltGr c o --> ǒ (Ŏ in HTML)
There is an article at linuxquestions.org that describes how to set this up.
The howto article refers to a wikepdia article which says
Gtk/Gnome has Ctrl-Shift-U, followed by the digits of the unicode character, then enter. Additionally “—” may be available in the standard keyboard layout for your language, or an alternative one. Mine is at AltGr-Shift-4 for example;