Prototype and scriptaculous are bundled with the Rails framework, and if you're not on rails or prefer jQuery, it along with the aforementioned can be easily included by linking to a copy of the code from google. See the google code page for more details.

One of the advantages of linking to the google code version of a javascript framework is that as more developers choose to do so it's likely that the client will have the code cached, thus allowing for the faster loading of your site's content as the JS library will be skipped from being downloaded again.

The latest development in JS library news is that Microsoft is jumping on board. Typically when Microsoft goes about implementing new functionality to it's existing tool set they build their own implementation or change an existing library before allowing it to exist in their development tools. This time though, they took a different approach. As can be seen on Scott Guthrie's blog...

"Providing the ability to perform selection and animation operations is something that a lot of developers have asked us to add to ASP.NET AJAX. As the team started to investigate building it, though, they quickly realized that the jQuery support for these scenarios is already excellent, and that there is a huge ecosystem and community built up around it already. Rather than duplicate functionality, we thought, wouldn't it be great to just use jQuery as-is, and add it as a standard, supported, library in VS/ASP.NET, and then focus our energy building new features that took advantage of it?"

This is truly great news and is one of the first open source projects that Microsoft has taken and implemented as-is into their tool set. I'm looking forward to being able to use the same rich animations and easy DOM selections that I've been able to use in rails for the last few years. The kicker... they're also providing intellisense!

The jQuery intellisense annotation support will be available as a free web-download in a few weeks, we'll keep you posted, but in the mean time as Scott Gu noted: "you might want to check out John Resig's post on the jQuery team blog.  Scott Hanselman is also about to post a nice tutorial that shows off integrating jQuery with ASP.NET AJAX."