SIGMA EX 15mm f/2.8
(diagonal) fisheye 180°
Sigma's 15mm diagonal fisheye is a fantastic lens. The 15mm perspective is wide. Really wide. It catches almost everything, even a stray camera strap.
With an omnibounce, most flashes work nicely with this lens, although in a large room it needs to be a powerful one. In the first image below, I used a Speedlight 430EZ.
The lens is good for close-ups, too, having a minimum focusing distance of 15cm.