Blizzard's Brood War Page: What StarCraft site would be complete without a link to the official page?
The Antioch Chronicles: This campaign page is really quality. If you want a campaign that uses StarEdit to its fullest potential, this is the place to go.
StarCraft.org: Probably the most professional StarCraft site out there. They have basically everything.
Campaign Creations: A big page that contains several .scm campaigns.