The largest is RSNA, held every year in Chicago in late November/early December. I believe it is the largest medical conference in the world. The other large general radiology conference is ARRS, which changes locations every year, but was held just couple of weeks ago in Vancouver.
The rest are mostly subspecialty specific (e.g. SSR, SPR, ASR, ASN), but there are many, many imaging conferences every month - many of which are designed for CME. Try Google; Aunt Minnie has a nice list as well.