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.

Department get-to-together, Sweden 2003
some Photoshop magic applied