Django or Ruby on Rails
Since you're a seasoned .NET programmer, you really should look at ASP.NET and MVC before turning to Rails and Django, at least if your goal is to get up-and-running quickly.  It's always easier to leverage something that you already know in-depth.
I was in a similar situation recently -- I had a project that I wanted to push out on Windows and Unix, and so rather than leveraging what I know well (Ruby/Rails), I jumped ship and gave Groovy on Grails a go.  It was a really rewarding experience, and I'm really glad I did a few small projects in Grails, but in the end I had a tiny fraction of the productivity I did in the environment where I had a few years' experience, even after months of hacking away on Grails.
So, pick up Django, Rails, Lift, or whatever you feel like doing -- it will help expand your mind, and give you different ways to look at solving problems.  But if you want to get going quickly, use what you know (.NET) on a platform designed for you (MVC).
I'm a C#/.NET developer looking to mess around with something completely different - something LAM(*) stackish for building web apps quickly.
I'm thinking either Django or Rails. I kind of like the Python language better and it seems to be more full-featured than Ruby for statistical, scientific and networking (let me know if you think this is wrong). However, the RoR community seems to be much bigger - which might make it a safer bet. Do you think this is important? Anybody have experience with both RoR and Django?

Update: Specifically I'm looking to be able to build a community finance oriented site quickly. There are definitely a lot of Ruby Gems to speed up the process, but I noticed Pinax on the Django side which looks promising. 
I know that Python is already fairly popular for financial/mathematical programming. Anyone else have an opinion?

Update 2: Noticed some comments about ASP.NET MVC. I have in fact done pretty significant work with ASP.NET MVC - a LOB app for Medical Equipment Servicing, and I loved it. It is IMHO a much better and more intuitive way to write web apps compared with ASP.NET web forms. But, I'm really looking for a non-.NET development environment to develop some chops in.