There are two versions. Halo: Combat Evolved and Halo: Custom Edition.
Halo: Custom Edition download:
http://hce.halomaps.org/index.cfm?fid=410
Patch:
http://hce.halomaps.org/index.cfm?fid=4987
You can just download a crack version for Halo: Combat Evolved.
But you would need a Halo Combat Evolved cd key to activate both of these games.
To answer the server question, we do. But not all lot of people are on them. They are pretty much dead with one server having some activity.