Django or Ruby on Rails
i'm another C#/.net guy with the same itch to tinker. Django is appealing. Though the development is easy and fun, i've heard some stories about deployment and configuration headaches with both Rails and Django (mostly rails). It's not like just throwing an asp.net app at IIS or PHP at Apache. For those things you can set it and forget it. I'd be curious to get c.batt's comments on his/her experince with deploying django. 
Lately I've been looking at Grails. It uses Groovy which is ruby like. It's very easy to develop in and you can build apps as fast as Rails but you get access to the Java platform via the JVM which could come in handy. There's a ton of open source java components you could drop into your Grails app, plenty of the types of components you'd be looking for. It's really not like building a java app. So far it does feel like Rails. Good luck.
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.