Link Grammar Parser seems like a really cool library: it parses English sentences, and seems to be quite capable. Looking at the sample parsed sentences, it can distinguish good and bad sentences that have completely similar structure (asterisk means it doesn’t parse):
The big question on everybody’s mind is who killed OJ.
*The big mind on everybody’s question is who killed OJ.
*5 millions attended.
Millions of people attended.
I don’t have a need for parsing English like this, but reading about a tool like this makes me want to try it out.