We work hard to keep our external libraries as up-to-date as possible with each major release. Here’s what you can find today in trunk, all of which are the latest version:
Edit: jQuery 1.8.1 (due out this week) and jQuery UI 1.9 (due out next week) are now both in trunk.
And, Backbone.js 0.9.2 and Underscore.js 1.3.3 were both added to core last week.
If you’re a plugin developer using a bundled library, we strongly suggest you follow their development as well, to ensure code you write will be compatible in the future. The projects we have adopted have very similar backwards compatibility policies as ours (which would be strive to always be compatible), which make this pretty easy to do.
Additionally, Prototype and script.aculo.us have been removed from core. They are still registered; enqueueing them will pull from ajax.googleapis.com. This is only being done for backwards compatibility and doesn’t indicate a change in policy. It was just a lot of bloat to keep shipping when core hasn’t used either for four years. (Both were updated to the latest versions in the process, something we can/may continue to do.)
See also the conversion to .min.js from .dev.js.