Django - Until 1.2 Django was a total headache but with 1.2 it really is a great tool to develop web applications. No reason not to use it unless you feel something's missing. Django has an amazing community with very helpful people.
web2py - web2py lets you create apps very fast, it doesn't push you hard and you can build something in a very short time. However when things get complicated, you will want to quit web2py. I don't like its form helpers, I don't like its validation as the validation is unfortunately not per object but the input data and url routing is headache with web2py. Web2py also has a great community. All your questions will be answered in a short time.
turbogears - turbogears is a subset of pylons with sqlalchemy as orm and genshi as templating system. i don't like the @expose decorator. it's just not for me. provides many other tools by the way. TurboGears has a small but helpful community. SourceForge is using TurboGears.
werkzeug - now this is the framework of my choice. the endless freedom. it is only a skeleton which lets you dress your application onto it very easily. you can choose anything as your orm and templating system. provides a strong url routing mechanism. supports sessions, secure cookies and any other thing you may need. however i am not using werkzeug anymore as i don't like the way people act in #pocoo on freenode. this is the only reason i am not using werkzeug. they are ignorant and i don't like it.
Pylons - My favourite Python framework of all times. Flexible as much as it can be. Provides you the freedom you need for anything. I strongly recommend using Pylons. I am currently using Pylons and I'm very happy with it. Pylons is also used at some very popular web sites such as bittorrent.com, reddit.com, etc. It has a helpful community whenever you need.
Frameworks aside, how will using Python will help you? * Very clean and beautiful code. * Easy updating and maintaining the code. * Easiness and laziness. * Great set of libraries. * Python for your mental health.
And I'd like to thank justin_vanw for the great sum. :)