The latest version adds many more editing features, as well as saveload and audio export.
Controls

Click and drag the mouse to draw lines, or to move line endpoints.
ZX deletes a line. If your mouse is over a line endpoint, it'll delete that line, otherwise it'll delete the newest line.
Leftright changes line sounds. If your mouse is over a line endpoint, it'll change that line's sound, otherwise it'll change the sound for new lines.
Updown changes the interval between drops.
E to startstop audio export.
S to save to the clipboard. In the web version, you'll need to press ctrl-C after this to complete the copy. (Or, on a Mac, ctrl-C so PICO-8 notices and then cmd-C.)
R to restore from the clipboard.
N to get a new empty scene.
When you paste a saved scene into PICO-8, Tiny Drops will automatically load it. In the web version on Mac, you'll need to press cmd-V to paste and then ctrl-V to get PICO-8 to notice the paste.

Known issues

Can get slow if you trap too many drops on the screen.
When you start to create a line, you may hear 'phantom' bounces until you start dragging the mouse. (Happens with 0-length lines.)

500-Character Jam Version
This version has no saveload, no audio export, and no concept of line hovering or editing existing lines: