Hobo looks great! It's a web framework built on top of Rails. It's structured as a plugin, so you can use as much or as little of it as you want.
I recommend checking out the screencasts to get a good overview of how this all works. I must say, I'm impressed.
I love the way it simplifies updating of several parts of a page with a single ajax call. Their solutions seems to be to tag parts of the page, then in the ajax call pass a list of parts that will need to be refreshed. The page is therefore telling the controller what parts to spit back, so the controller doesn't need to know the 'current page'. I'll have to look into exactly how this works.
I was also very impressed by DRYML, their templating system. It's just so simple! You define custom tags which is equivalent of defining functions, so they have a list of parameters and a body. When calling the tag you can pass in the details as the parameters. There is a nice concept of a context object for evaluation of any part of the DRYML, which simplifies the code in the tags too.
[It still find it amusing when people proposing their framework as the fastest way to develop a web app use php for their own site. What's with that?]