I just found out about this (thanks to LifeHacker), but MyCorporation is offering free incorporation (C, S, or LLC, among other options) through Dec. 5, 2008. You'll still have to pay the fees required by your state, but the regular $149 service charge has been waived. If you've been thinking about taking the step, this could be a great opportunity, and there are even some screens you can walk through to help figure out which is the best business structure for you. It's easy to start the process, and it might be the logical next step in your small-business success.
Incorporation isn't the right path for every business, but if you've been rolling as a sole proprietor and wondering if you should change your business structure, this could be the opportunity you need to explore something new.