LinkGrammar: http://www.link.cs.cmu.edu/link/ is the other NLP tool kit in python, may be suited for use by those who might be intimidated by the need to write their own grammar.
LinkGrammar looks very interesting and useful. But I believe that its scope is somewhat smaller than NLTK. Aside from sentence parsing, NLTK also gives you a bunch of tools for document tokenization, statistical analysis, classification, and some easily accesible corpora to try it all out on. I'd almost call it a natural language prototyping environment; it's a huge time-saver to those dabbling in the field or who need to quickly experiment with different techniques.
It is integrated into and maintained by the authors of Abiword. There is a talk on this other library at Pycon, this year: https://us.pycon.org/2012/schedule/presentation/187/