Although it is quite a bit more complicated than the United Kingdom extended keyboard that comes with Windows, it allows you to type quite a few more characters with some relatively logical keystrokes.
My favourite is [Alt Gr]+[`], [letter] to type Greek letters. So [Alt Gr]+[`], [s] allows you to type the letter “σ”. For all of you writing technical papers with equations, this is, in a word, brilliant!
The download includes an html file that describes what keystrokes are necessary to type the character you want.