Halo.Bungie.org (HBO) has just finished uploading all Halo 2 cutscenes to their servers in gloriously trendy high definition.
Halo fans and machinima enthusiasts take note: all of the assets are available for standalone download (none of this embedded crap) and ready to be edited as you see fit.
The entire Halo 2 cut scene library here
Halo cut scene library here