I've used the Manfrotto Justin Clamp as long as I can remember, and until I bought my first SB900 they served me well. Frustratingly, the SB900 has an ever so slightly thicker shoe and once jammed onto a Justin clamp it's nearly impossible to get it off (I'm not sure if newer models of the clamp have compensated for this so you may want to check before buying). I came up with a quick solution and in the end I've ended up with a speedlight clamp I'm even more happy with.

The original Manfrotto Spring Clamp with Flash Shoe (aka Justin Clamp) comes in once piece, but you can buy just the standalone Bogen 2936 Clip Clamp and then add my recommendations for a ball head and cold shoe.

I use the Giottos Professional Ball Head Camera Mount. It's sturdy enough for my speedlights and has a fully adjustable ball that also pans 360 degrees.

The original Manfrotto shoe is plastic, and although strong enough, it doesn't have a recess in the shoe to provide space for the flash terminals. I replace it with the Stroboframe Accessory Shoe. It also has a sturdy adjustable side clamp that assures a snug fit with any of my flashes. The Stroboframe shoe simply screws onto the Giottos Ball Head and you're ready to go.

To accommodate the new allen bolt, you'll need to make the mounting hole on the clamp slightly larger, so you will need a drill and drill bit as well.

From a hardware store:

1/4-20 x 1/2" long allen head bolt

1/4" washer