In case you don't know, some time ago I started a ruby-on-rails plugin for RightJS called right-rails, which was just updated to the next version 0.3.0
There are actually quite a lot of interesting and fancy stuff going on and it tries to solve many of usual headache of ajax applications development.
Like say it transparently handling files uploading for remote forms, so that there is no difference if you have or have no files on your form. Plus there is a simple javascript interface for the most common ajax operations which is super easy in use.
Then there is a new much more powerful RJS scripting replacement, that allows you write javascript in ruby and more of that mix in one flow the serverside and browserside contexts, it has automatic types conversion and lots of other things.
Then small things like transparent Prototype/Scriptaculous helpers replacement for RightJS, plus various helpers for the RightJS own features and modules, plus automatic scripts including handler, etc. etc.
If you like RightJS and do ajax stuff in rails, check it out, you'll find many helpful stuff.
