Or the Canon IR ST-E2 (low option - too expensive)
FYI The Sigma EF500DG Super "becomes" an ST-E2 when you turn its flash output off! (ie you automatically get the ST-E2 when you get the Sigma). It does this through the pre-flash signaling just like on the 550EX...
There's also a hidden feature in the Sigma in that (like a Metz) it can control others Sigma's wirelessly through it's own protocol. You can set theses Sigma's just like the Alien Bees each with their own ratio and they all work wirelessly independent from the Canon's E-TTL (ie no pre-flash here) and syncable with the AB as well (page 23 - Wireless Manual Flash)