These aren’t the easiest to find, but here’s some that are useful!
(Taken from the osu! wiki – page found here)
I frequently use…

• Ctrl + Alt + Shift + S
• F7
• F8
• F10
• CTRL + O
• F12 / SHIFT + F12
• Alt + Arrow up/down (volume while in song select)

Osu!taiko; osu!ctb; osu!mania; Performance Points Display. Don't show; replace level; after accuracy; above rank; Display your country rank (experimental) Your Signature. BBCode: Generate Advanced options. - Rhythm is just a.click. away! With Ouendan/EBA, Taiko and original gameplay modes, as well as a fully functional level editor. Osu!mania is based on a variety of conveyor-type rhythm games (namely Dance Dance Revolution and Beatmania). The game mode was chiefly introduced and implemented by woc2006, and is made to mimic the feel of a real piano through the use of keyboard keys. Last update was done 0d 12h 45m ago. Total records stored: 31348716. Created by Piotrekol and EzodaPiotrekol and Ezoda.

Hotkeys below!

There are many shortcuts in osu! that can be utilized while in-game or while idle at any screen.
They are listed out with their functions below.

Osu Mania Download

Note: some of these keys can be changed in the options!
However, the list below presents the default keys.

Game-Wide

These shortcuts work anywhere:

• Ctrl + Alt + Shift + S: Reload current skin
  • If playing or at the results screen, this will queue the command to be executed after returning to song selection.
• Ctrl + F11: Toggle frame times
• F7: Toggle frame rate limiter
  • Pressing this will cycle through 120fps, 240fps, and Unlimited (gameplay) but not enable VSync.
• F8: Toggle chat window
• F9: Toggle extended chat window
• F10: Toggle all mouse buttons (except for mouse wheel)
• F12: Take a screenshot
• Shift + F12: Take a screenshot and upload to the server for sharing
• Insert: (Boss Key) Minimize the osu!client into the notification area
• Shift + F1: Open osu!status (when connection error appears)

Main Menu

• Alt + F4: Close the osu!client (no prompt or dialog)
• Alt + Enter: Toggle fullscreen mode
• Arrow Up: Increase volume
• Arrow Down: Decrease volume
• Esc: Go back one menu
  • Close options, if open
  • Go back to main menu, if in play menu
• P: Play menu
• P, then P: Solo
  • alias: E, then P, then P
• P, then M: Multi
  • alias: E, then P, then M
• P, then E: Edit mode
• D: osu!direct (osu!supporters only)
• O: Options
  • alias: Ctrl + O
• Q: Display exit dialog
  • alias: Esc, when the options and play menu are closed

Jukebox

• Z: Previous song
  • alias: F1
• X: Play from start
  • alias: F2
• C: Toggle pause
• F4: Stop playing and reset duration to 0
• V: Next song
  • alias: F5
• J: Jump To
  • alias: F6
• R: Random song

Song Select

Note: Not all of these work in the multi mode’s song select screen.

Osu Mania Online

• Alt + Arrow Up: Increase volume
• Alt + Arrow Down: Decrease volume
• F1: Mod selection menu
• F2: Randomly select a map
• Shift + F2: Undo random map selection
• F3: Beatmap options
• F5: Reprocess beatmaps (may display a dialog)
• Shift + Delete: Delete selected map
• Ctrl + 1: Switch to osu!
• Ctrl + 2: Switch to osu!taiko
• Ctrl + 3: Switch to osu!catch
• Ctrl + 4: Switch to osu!mania
• Ctrl + Enter: Play current map with Auto mod
• Ctrl + Shift + Enter: Play current map with Cinema mod
• Ctrl + A: Toggle Auto mod
• Ctrl + F3: Increase osu!mania speed (only in osu!mania mode)
  • alias: Ctrl + +
• Ctrl + F4: Decrease osu!mania speed (only in osu!mania mode)
  • alias: Ctrl + -
• Ctrl + Backspace: Delete last word (when using search)
• Up: Move selector one beatmap up
• Down: Move selector one beatmap down
• Left: Change selected beatmapset one up
• Right: Change selected beatmapset one down
• Page Up: Scroll up
• Page Down: Scroll down
• Enter: Select selection or play selection
• Shift + Left Arrow: Change selected group one up
• Shift + Right Arrow: Change selected group one down
• Shift + Enter: Toggle selected group

Game Modifiers

Main page: Game Modifiers

Osu Mode Osu Mania

Notes:

• The key references below assumes that none of the mods are selected.
• Some mods may require the player to press the key twice.
• Q: Easy
• W: No Fail
• E: Half Time

• A: Hard Rock
• S: Sudden Death
• S, then S: Perfect
• D: Double Time
• D, then D: Nightcore
• F: Hidden (osu!standard, osu!taiko, and osu!catch)
  • this will toggle Fade In for osu!mania
• F, then F: Hidden (osu!mania only)
• G: Flashlight

• X: Relax
• C: Auto Pilot
• V: Spun Out
• B: Auto
• B, then B: Cinema

• 3: 3K (osu!mania only)
• 4: 4K (osu!mania only)
• 5: 5K (osu!mania only)
• 6: 6K (osu!mania only)
• 7: 7K (osu!mania only)
• 8: 8K (osu!mania only)
• 9: 9K (osu!mania only)

Play Mode

• + or -: Adjust local offset (hold Alt to change the precision)
• Esc: Pause song (or quit if Auto or Cinema mod is used)
• Space: Skip opening cut-scene
• Tab: Toggle scoreboard
• Shift + Tab: Toggle in-game interface
• H: Toggle scoreboard and replay interface (only available when watching a replay or spectator stream)
• F3 or F4: Adjust speed for osu!mania (only works near the beginning of playing)
• Ctrl + R or ` (hold briefly): Quick retry the current map (autoskips the intro upon retry)