Key and Tempo
While a song is playing you can press ⌘-[ or ⌘-] to lower or raise the pitch, respectively. Or press ⌘-\ to re-set it to the original pitch. This does not affect the tempo.
Also, you can press ⌘⇧-[ or ⌘⇧-] to slow down or speed up the tempo, respectively. Or, press ⌘⇧-\ to reset it to the original tempo. This does not affect the pitch.
These items are also available under the Controls menu.
Pro version only
If the song belongs to a singer, then that setting will be remembered permanently. You can also set the pitch via the popup menu listed with the song itself.