Django or Ruby on Rails
If you want a quick stack check out Sinatra.
Django will make more sense faster. It has awesome routing, simple file structure, nice clean syntax, not a lot of WTF moments, and understandable ORM functionality.
Rails you will have to dive into. You will battle activerecord. You will try to wrap your head around the many facets of a rails application.
I like them both. Try them both out. They don't take that long to get a feel for. You should ultimately base your decision on a per-app basis and factor in which language has better modules/extensions/gems for what you need.
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.