How do you type the Unicode '§' symbol in Minecraft?

It looks likes you have two options:

There is an ALT Code you can try to type. To try it:

• Hold down the ALT key on your keyboard
• Using the number pad (it must be the number pad), type 2, then 1 (21). Make sure Num Lock is on.
• Release ALT and it should type a §

On Mac, it looks like if you hold ALT and type 6, it will type the § sign (ALT and 5 for extended U.S. keyboards).

The other way is to use plugins that replace the § with something else. Looking around, it seems CommandBook for Bukkit may work.

Sources: Minecraft Wiki, Talk:Formatting codes

Minecraft stopped the Alt code insertion a long time ago. At one point they allowed insertion of Unicode characters in command blocks, but that too was removed. The only way now is via external editors, such as NBTExplorer.

Other than external editors or Bukkit plugins(Essentials v3 supports using & instead), I do not think there is any other way to insert section symbols.