NOTE: I included a lot of links to frameworks, projects and what-not, but as a new user I was limited to 1 link per answer. If someone else with enough reputation to edit wants/can edit them into this answer instead of the footnotes, I'd be grateful.
There are some Python IDE's such as Wing IDE[1], I believe some people use Eclipse[2] with a python plugin[3] as well. A lot of people in the #python channel of FreeNode seem to prefer vim, emacs, nano and similar text editors in favor of IDE's. My personal preffered editor is Vim, but if you've mostly done .NET development on windows, presumably with the usual Visual X IDE's, vim and emacs will probably cause you culture shock and you'd be better of using an IDE.
Nearly all python web frameworks* support the WSGI standard[4], most of the large web servers have some sort of plugin to support WSGI, the others support WSGI via fast cgi or plain cgi.
The Zope[5] and Django[6] frameworks have their own ORM's, of other ORM's the two most well known appear to be SQL Alchemy[7] and SQL Object[8]. I only have experience with the former, but both support all possible sane database choices, including SQLite which is installed together with Python and hence perfectly suited to testing and experimenting without polluting your .NET environment with 3rd part web servers and database servers.
The builtin unittest[9] and pyunit[10] frameworks seem to be the preffered solutions for unit testing, but I don't have much experience with these.
bpython[11] and ipython[12] offer enhanced interactive python shells which can greatly help speed up and testing small bits of code and hence worth looking in to.
As for a list of well known and often used web frameworks, look into the following frameworks**:
- Twisted[13] is a generic networking framework, which supports almost every single protocol under the sun.
- Pylons[14] is light-weight framework aimed at being as flexible as possible and leaving all the choices about what ORM, templating language and what-not to you.
- CherryPy[15] tries to provide an interface to expose Python objects to the web.
- Django[6] attempts to be an all-in-one solution, builtin template system, ORM, admin pages and internationalization. While the previous frameworks have more DIY wiring together various frameworks work involved with them.
- Zope[5] is aimed to be suitable for large enterprise applications, I've heard nothing but good things about it, but consensus seems to be that for smaller you're probably better off with one of the simpler and smaller frameworks.
- TurboGears[16] is the framework I know the least about, but it seems to be mostly competition for Django.
This is everything I can think of right now, I'll edit and add stuff if I can think of it. I hope this helps you some in the wonderful world of python.
* - The main exception would be Apache's mod_python, which you should avoid for exactly that reason, use mod_wsgi instead.
** - Word of warning, I have not personally used these frameworks this is just a very short impression I have gotten from talking to other people about each framework, it may be wildly inaccurate. (If anyone has any corrections, do comment and I'll try to edit and fix this answer).
(The http:// is missing since they're recognized as links otherwise)
[1] www.wingware.com/
[2] www.eclipse.org/
[3] pydev.sourceforge.net/
[4] wsgi.org/wsgi/
[5] www.zope.org/
[6] www.djangoproject.com/
[7] www.sqlalchemy.org/
[8] www.sqlobject.org/
[9] docs.python.org/library/unittest.html
[10] pyunit.sourceforge.net/pyunit.html
[11] www.bpython-interpreter.org/
[12] ipython.scipy.org/
[13] twistedmatrix.com/trac/
[14] pylonshq.com/
[15] www.cherrypy.org/
[16] turbogears.org/