The list includes the likes of
1. jQuery by John Resig (jquery.com)
2. Yahoo User Interface or YUI (developer.yahoo.com/yui)
3. Dojo (dojotoolkit.org)
4. Prototype js (prototypejs.org)
Traditionally, all the script tags were placed in the head section so as to keep all the external files references(js + css) in one place. However, including all the script tags in head section would mean that all the script files have to be downloaded and parsed and interpreted before the page starts rendering which can easily cause delays and performance of the website takes a beating. So the ideal place for the script tags is in the body section towards the end of the page.