The Custom Mapping Team is considered a legend amongst many mappers. Although CMT was often mistaken for ripping due to the beauty of their work, they always (although there was a time when they ripped from H2) make it themselves (sometimes with other people outside the team), and even modifies some of the tags, so the tags aren't exactly H3 copies, hence custom.
On March 1st, CMT broke up and Masters1337 (team leader) released all their content including some of their unfinished unplayable campaign.
They however have since reformed, and are skipping finishing Single Player Version 2 (SPV2), and are working on thier 3rd iteration of the Halo Combat Evolved Campaign, aptly named SPV3. They released a beta version of the third level, Truth and Reconciliation, and while not 100% complete and polished, recieved a generally positive response, since they were catering for those who did and didn't use Open Sauce by releasing separate versions.
They are currently still working on the other levels of the campaign.
CMT has made many custom maps, most of which have been admired by many players.
SPV2 (incomplete and discontinued)
SPV3 (currently in progress)