ya HMT(Halo Mapping Tools) is used to mod regular halo maps by swapping something with another thing. There is Sparkedit for moving different things and adding vehicles and scenery and startpoints etc in a Halo PC map(not sure if it works for CE maps as well
and HEK/Gmax-3DSmax 7(emphesis on the seven) is for creating custom CE content.
I can prolly track down all of those if you want so PM me and ill see if I can get them.(used to be in [MT] which is Modtacular and its a modding clan)
There is als another program thats escaping my mind atm that you swap dependencies in it and itl let you do some cool things that HMT wont let you do.