Are there any good web frameworks for C?

There is GWan (http://gwan.com/) but it is not open source.

If you are OK with C++, there is Wt (http://www.webtoolkit.eu/wt).

https://github.com/AlexBio/Tofu is pretty awesome (but also pretty new)

Probably not, and you will be doing so many string manipulations that it might not be worth trying. There are a few libraries for getting CGI parameters, but that's all I'm aware of.

If your reason for wanting to code in C is for performance at a popular site, cacheing and server configuration will probably be the most important factor.