Copy and paste the source code (linked in the titles) prepended with javascript: into your bookmarks and open the bookmark to activate it.
For example, the source code function example() {}; should look like javascript: function example() {}; when saving it as a bookmark.
Note that character limit, comments, and special characters may be an issue when creating a bookmarklet.
If there is an issue, try running it through JSObfuscate first.

View a webpage in 3D layers.
Includes a settings menu in the top left corner.

Add a piano to any webpage.
The left menu allows you to adjust:

• wave type
• octave level
  • can also use left and right keyboard arrows
• axis tilt
  • can also use up and down keyboard arrows

The right menu has the following buttons:

• [# ▶]: play button
• [# ◼]: stop button
• [↻ #]: history playback button
• [✖]: delete a disc or clear history
  • can also clear history with spacebar
• [⬇]: save history as a a disc
• [⬆︎]: export all saved discs
  • only useful with a local copy of the source code (e.g. me and pull requesters)

Simulate a flashlight. Works best on websites whose HTML elements are deeply nested.

A game of ultimate tic-tac-toe.
The rules are explained here.

A replicated Lumosity game.
See Lumosity's pinball recall.

Preview the likes-to-views ratio of YouTube videos.
For easier visibility, the ratio bar is displayed as log(likes + 1) / log(views + 1).
This is more useful with a userscript manager like TamperMonkey.

Watch a video and read comments simultaneously.
This is more useful with a userscript manager like TamperMonkey.

Keep a time history so you can return to normal viewing after clicking a timestamp.
Recommended to pair with YTScroller.js due to its scrollAnchor function.
Not recommended to pair with YTLiveStamper.js due to overlap.

Record timestamps during a livestream.
Here's an explanation of the parts of the YTLiveStamper panel:

• Stamplist
  • left side has clickable timestamps with copyable urls
  • right side is for notes and comments
• "End of Video" Timestamp
  • always at the bottom of the stamplist
  • it's there so you can return to real time after checking a past timestamp
  • usually useless since most live streams don't allow backtracking in the first place
• "Import List" Button
  • reads textbox to create new stamplist
  • useful if you need to refresh the page or adjust timestamp times
  • note: overwrites stamplist
• "Add Timestamp" Button
  • adds current time (minus 5 seconds) to stamplist
• "Copy List" Button
  • writes stamplist to textbox
  • copies textbox to clipboard
  • double-click to copy stamplist as list of urls (cannot be imported)
  • note: overwrites clipboard

Toggle picture-in-picture mode with one click.