I'm intrigued by the Ruby on Rails framework created by David Heinemeier Hansson and the productivity gains that it promises. Dave Thomas of Pragmatic Programmer fame has coauthored a book with David about Ruby on Rails titled Agile Web Development with Rails. They've released this book as a "beta" book in order to get it out sooner, and Beta2 has just been released.
There's a lot of hype being generated around it, but Brad Wilson doesn't seem to buy it. I'm reserving judgment until I've taken a look at it in more detail, but it looks interesting.
Update: Brad let me know in the comments that he has worked through the installation and set up issues with Ruby on Rails on Mac OS X and is enjoying Ruby and Rails. His detailed follow up is here. Also, the final beta of the Agile Rails book is available. The final version of the book should be available by late July/early August.