I want to use tags for a model in my Rails application, and I'd like to have the same kind of tag selection mechanism that Stack Overflow has when asking a question. I.e. I type in 'rails' and it drops down a list of tags that include 'rails', and I can go on to add more tags from there. Is there a plugin that can do this?

I'm comfortable putting together the tag model and migrations myself, that isn't an issue. I just need something to handle the AJAX and javascript. I'm using prototype and Rails 2.3.