Django is one of the most famous. It follows a different approach to web devlopment then ruby does, but it is just as powerful and feature rich. An example website running Django is lawrence.com
Pylons is another popular one, I don't know why you heard it was a Rails clone, because it is not. It is a lightweight framework that leverages the power of other open-source projects to give you flexibility in implementation. For example, you can choose to use SQLAlchemy, SQLObject or CouchDB for managing your database. Or you can choose between Mako, Genshi, Jinja2, or whatever you like for your templates. I think you get the picture. Some example website running of pylons are: freebase and Charlie Rose
There exist other web framework as well, but they are less popular.
Notably, TurboGears, which is now built upon Pylons. I would say it tries to pack more juice then pylons does, but it also constrain you more as it assumes more decisions for you. Still, you can stay away from them and do as you please, but it starts with a more constrained framework.
The last one I will mention is Zope, which is the big commercially backed one, that has been there for a while now, but I don't have much experience with it. I do believe it is the less "fun" to work with, but that's just my feeling, you can check it out yourself.
All in all, it comes down to your workflow, I personally, do not enjoy Ruby as a language as much as I do Python and it is natural that I thus like to work with python for web development then Ruby. You really need to try them out yourself, at least the first two I mentioned, try to build a small website, just to get a feel for it. All I can say is from my experience, people either like Rails or Python, not both...
Good Luck!