Ruby on Rails has to be the one of the real breakthroughs in Web Development and indeed, development itself. Though I've only spent a trivial amount of time playing with it, the huge productivity hike and the overall elegance of the framework is enough to convince me that this is something to pay and devote attention to.
This is no hyperbole. Far more so than Ajax, this is this era's true revolution in web development. The majesty of the framework is sufficient for me to make a long term commitment to Ruby; In fact, calling RoR a Ruby framework understates things; it's probably the killer app for Ruby itself.
The only downside to it is having to move back to a world without RoR…