Probably the most well known is EMC, with its Centera platform that has had more than 4,500 customers to date. It has been out since about 2002. The other big players have jumped into the CAS pool; IBM with the DR550 and Hitachi Data Systems (HDS) with the Content Archive Platform. But probably since Centera has been out the longest, none of them quite have the traction that Centera has.
There are also some smaller players. Plasmon offers CAS with optical disk, and optical disk has its pluses and minuses for active archive uses. Nexsan has some traction with its Assureon product, which provides a good entry-level product for SMBs.
Caringo, a startup that is lead by the original CAS inventors that worked for FilePool when EMC acquired FilePool, offers a software-based platform that can run on commodity server hardware. But interestingly enough, most of the competition we see when Centera is involved in a deal comes from block network-attached storage (NAS), such as NetApp or Sun.
Check out the entire CAS FAQ.