Xdesktopwaves, waves following your mouse cursor. It also supports raindrops and other effects, but I just want to show 1 for now. Also, this can consume quite some CPU, as you can see on the system monitor on the right.
But nice, it is!

Should be available in Synaptic. Might not work with a desktop that replaces the native X background.

Try icewm or openbox for a test.

Example usage:
xdesktopwaves -t -q 5 -wmo 40 -rain 0 -cs 2 2 -fr 14 -li 10 -si 10 -wc "#000000" -sc "#000000" -nww -fr 30

Seems to look nicer with active composite manager (compton -c), but this takes more cpu usage.