I am looking for any information on hosting companies that run .NET/MS SQL Co-location servers. Currently, the only one I know of is RackSpace. What would be there competitors?

Colocation? If your colocating you can run/install anything you like.
Don't you mean dedicated servers?

Yes, but I want to have someone who will manage the servers for me, too.

David,

COLO is when you send your own hardware to be colocated in an datacenter (Rackspace dont offer this).

DedicatedServer on the other hand, is when the supplier provides hardware etc.. etc.. and then value added services on-top such as Managed Service ... (Rackspace offers this)

Make your mind up on what you need first...