Here's my initial thoughts on the ones I've tried out:
- MooTools - Looks nice, but severely lacking in beginner samples. I couldn't even reference the fully loaded MooTools.js (v1.1) file without throwing errors. Needless to say, this did not go over well with me. It's a real shame as I like this tool set over Rico.
- Rico - Speaking of which... Rico was the first framework I ever saw and it just blew me away. I've never had a chance to implement it and, to be honest, I had initially shied away from it because I wanted to pick something that had a higher user adoption rate. I've still not tried it as I'm told that it is an extremely bloated framework and there are much lighter frameworks (MooTools for instance) that put Rico to shame. That makes sense as, to my knowledge, Rico is essentially an effects package that sits atop the Prototype framework.
- Prototype - This is, to my knowledge, the (most well known) framework that started it all. It provides all sorts of hooks and utility methods that make common functionality a breeze and it also has the benefit of eliminating the need for knowing the intricacies of cross-browser scripting. Not only that, but it's the first one that I've looked at that actually worked. Granted, the examples I was looking at were still too meaty to be easily digested by a n00b like myself, but I was able to supplement their examples with examples found in the blogosphere. Once again, another reason to make wide user acceptance a criteria for picking your framework.
- Adobe Spry - This is toted by Adobe as being the framework for designers and I can say by the lack of technical documentation that this would appear to be true. While it would be nice to pick a framework that is backed by a heavy-hitter like Adobe, I just can't get into a framework that doesn't have the documentation to make my learning process that much quicker.
- Yahoo! YUI - I haven't looked at it as Prototype just started working for me, but it's on my list of things to check out. Perhaps Andrew has taken a peek at it?