If you're in the web design & development work, you would have heard about it unless you've been living under a rock, figuratively speaking. jQuery was released in January 2006 and is licensed under the GNU GPL and MIT license. In other words, jQuery is open source and available for free!
The library itself is just 97kB and the latest available version is jQuery 1.2.6 (the previously available version is jQuery 1.2.3). You can read more about the changes made in the latest version here:
EDIT: As Rey Bango points out (in his comment on my blog mirror), the jQuery library is just 16kB when Gzipped. Since most modern browsers support GZip compressed content, the web servers (if configured correctly) should send the content in a compressed form.