Command and Conquer Tiberian Sun
Containing all of the maps submitted in the last Open Map Competition., it has a great variety of maps.
Here's a well-made series of maps that picks up where the original Firestorm Campaign left off, at Cabal's defeat.
TT's Map Pack is a rather large C&C: Tiberian Sun map pack that adds many new generic multiplayer maps.
This is the continuation of the Cabal's Return. It continues the campaign, although this campaign can also be stand-alone.