The BBC have released their own reusable JavaScript library as an open source project at http://www.bbc.co.uk/glow/. Previously, this library was for internal BBC use only and its opening up to the community is a good step in reaching out to web developers the world over.
The library itself clocks in at 84 kB minified and ticks most of my boxes for a worthwhile JavaScript library (see Chapter 2 of my book to read more about my criteria). I commend this action and recommend this library – its support by the BBC means the community is in good hands.