This is aimed at people writing tweet-sized JavaScript for http://140byt.es/ . This is a great resource for learning JavaScript and has some very impressive code behind it. Generally for production sites, though, I'd much rather go with jQuery due to its extensive test suite and bowser support. Between tweet-sized and jQuery-sized JavaScript, there's a whole lot of micro JS scripts / frameworks on http://microjs.com/

I'm currently writing a mobile web app that needs to work over GPRS connections with 30kb/s bandwidth and up to 1s latency. In these cases, shaving off a few bytes to get a page in 1 fewer TCP/IP packets really does make a difference.