I think everyone knows this but ...
In the Main Menu, if you press d, you can go to osu!direct.
F1: << Prev song played (Also z)
F2: Play (Also x)
F3: Pause / Unpause (Also c)
F4: Stop playing
F5: >> Next song (Also v)
F6: Jump To.. (Select song)
F7: Change Frame Limiter
F8: Bancho?
F9: Online players.
F10: Enable / Disable Mouse buttons
F11: ???
F12: Screenshot!
While you are selecting maps (Solo gameplay)
F1: Toggle mods (all of them are respectively Q W E = Easy No-Fail Half-Time; A S D F G = HR SD DT HD FL; Z X C V = Relax Autopilot Spun-out Auto).
Also, double toggle Double Time = Nightcore; double toggle Sudden Death = Perfect (Auto retry if you didn't get a 300); double toggle Auto = Cinemax (Play the song in auto, but now background dim = 0, no circle will appear. Enjoy the video and the storyboard!)
F2: Random songs (If you click it once, it points you to a random map instantly. If you hold it, the roulette spins longer!). This works even after you put on search (cs=4 ar=9 for e.g)
Talking about search parameters, you can put in cs, ar, od, bpm and even length! (yes, length<120 = beatmaps shorter than 120s).
And of course you can "combo" them: putting "cs=4 ar=9 insane length<120" in the search bar = practice day for meh!
F3: Same as right click on a song (useful for tablet user like me with option of "Enable OS TabletPC support" which merge left and right click on my tablet)
F4: ???
F5: Refresh osu! songs and beatmaps library.
F6: ???
F7: Frame Limiter
F8: Bancho
F9: Online players.
F10: Enable / Disable Mouse buttons.
F11: ???
F12: Sreenshot!
Also +1 to Nathanael about the Ctrl + Alt + Shift + S godlike combo. However I noticed that this only works for sprites (cursor, hitcircle, etc), but if you change something in the skin.ini, it still requires a quick change skins / osu! restart. Still, customizing a skin shall never be the same anymore!