Wily modders have recreated five of the six Starcraft campaigns within the Starcraft 2 engine. These are complete, faithful recreations of the original Starcraft and Starcraft: Brood Wars singleplayer game, including the original techtrees, dialogue, and even the briefing screens. These first five are available to download and play, and the modders have recently announced work has begun the final Queen of Blades campaign.
Blizzard have been moving at a glacial pace when it comes to releasing new campaigns to Starcraft 2 so it's a godsend that the modding community have stepped in to bring in the previous games singleplayer levels into the new engine. It relieves something of the pressure building up as we wait for the Heart of the Swarm beta to begin. They've done sterling work too, with just the final Zerg campaign from Starcraft: Brood War, Queen of Blades, to appear.
You can find all the previous campaigns at these links, which I pilfered from the mod page:
Head on over to give those guys a bit of support in the threads.
This is an impressive feat, I mean look at how well it's captured the original:
If that's sounding familiar it's because on top of all the revamping these guys have done, they've also downgraded the new sounds to the old Starcraft effects.