Technically, the only expansion to StarCraft are called StarCraft: Brood War (excellent expansion, though), and the only thing they are working on that's SC related is StarCraft: Ghost, a 3d person action adventure starring a Ghost unit. There are also a few StarCraft books out if you want to shorten the wait until StarCraft 2, which isn't even confirmed yet.
That said, there are tons of unofficial things out there worth checking out. Many people have made their own campaigns, some which are actually quite good. Some guy made a full StarCraft campaign complete with custom units and buildings. Never tried it, but it sounds great. Just do a quick search on the net, and you'll come up with tons of that stuff.