MochiKit 1.1 is now available [download]. This release contains a bunch of bug fixes, performance enhancements, cross-referenced and bookmarkable documentation, and some convenient new APIs.
Also make sure to check out the new MochiKit 1.1 Intro Screencast!
MochiKit is still quite lightweight, the entire library is just shy of 90k (up from 89k in the 1.0 release). Comparatively, this is the same size as the AJAX edition of Dojo Toolkit 1.0.
MochiKit.DOM.currentWindow and currentDocument allow you to change the global window and/or document object used by all MochiKit.DOM functions, which allows you to use mock DOM objects (great for testing), deal in a separate window, frame, or XML document's objects, etc.
MochiKit.Async.Deferred.prototype.addCallback, addErrback, and addBoth now accept additional arguments, which will create a partial function.
MochiKit.Async.doSimpleXMLHttpRequest will now use additional arguments to build a query string, via MochiKit.Base.queryString and/or MochiKit.DOM.formContents depending on the arguments given.
There's a new MochiKit.Async.DeferredLock object to use for synchronizing Deferred instances.
For a full list of changes, see Version History.