Ruby on Rails
Ruby is a modern programming language that is about economy of code, and productivity. It is fully featured, and a pleasure to code in due to it's pure object oriented nature. This also contributes to how easy it is to write DSLs in. Ruby could be considered a hybrid between Smalltalk, Lisp and Perl. Due to how many areas it pulls from, the language itself can take a little while to wrap your head around. It's very different than the "big" languages out there (C#, Java, PHP). I plan on writing a much longer post about Ruby specifically, but here are a couple highlights:
Probably the biggest distinguishing factor of Ruby is it's Gems. Perl started going down this route with CPAN, but Ruby took it all the way to its logical conclusion with Ruby Gems. Gems can be libraries, DSLs or executables. It could be something you use in your code, or a command you run to push code to a server. The biggest difference from the 'big' languages out there in this realm is that they can be installed with one simple command on the command link, unlike other library dependency managements systems that require you to either hunt down the dependencies, or download the internet every time you build. For example, pulling in the "rails" framework is as simple as: "gem install rails" and waiting a few minutes.
The other big highlight of Ruby that I'll make is the Rails framework. It's one of the most amazingly productive frameworks for designing your web application, and despite rumors, scales perfectly well. Assuming you know how to write scalable code.There are a ton of great tutorials out there on how to get started with Rails, so I don't ever really plan to cover Rails in any more detail (unless there are specifics I decide are worth covering). Suffice it to say, you could have a running application in about five commands on the command line. Which includes full database backing (with a database of your choice), basic unit tests for generated models and controllers, with proper separation of concerns between layers.
Sencha's ExtJS Framework
There are plenty of tutorials out there for both, so I wont belabor those subjects. But all in all? At this moment in time, I'm thinking that Ruby on Rails partnered with ExtJS is a match made in development heaven.