Original video via Jason Kottke.
In plain English, here's what's happening: The top of the Slinky is accelerating downward at faster than the rate of gravity; the middle of the Slinky is accelerating downward at roughly the rate of gravity, and the bottom is being pulled upward by the Slinky at about the same rate that gravity is pushing it down. That's why it stays still. No magic, just science.
Anyway, back to the GIFs: