This is a fun toy you can play with in your browser (including your phone's/tablet's browser). It's hosted at particles.johncs.com, check it out!
The important controls are (hopefully) discoverable but here's a list of everything you can do:
[click]inverts the mass of the mouse's gravity well[mouse move]moves the mouse's gravity well to the mouse[spacebar]resets all the particles[touch/drag]creates a gravity well at touch point[multi-touch/drag]creates a gravity well at each touch point
I made this in C back in high school (original source) but remade it in Javascript at some point while in college.