Ctrl+I - convert selected text to italic. The reverse operation is performed by pressing the buttons again.
Ctrl+U - underline the selected text. The reverse operation is performed by pressing the buttons again.
Ctrl+1 - Ctrl+4 - insert 1-4 non-broken spaces. If a text of some paragraph is fully justified, word separations are chosen automatically. However if you want to keep a separation between some words always the same, put a non-broken space instead of normal space between the corresponding words.
Alt+C - sets current color to selected text (available when the "HTML Colors" window is open).