If you are using Prototype as your javascript framework (even if you are not using it) and want some out of the box user interface effects you can try Scriptaculous (script.aculo.us).
It is an add-on for Prototype and is really light and handy. Try out the following links
site : http://script.aculo.us/
quick start guide and examples : http://wiki.github.com/madrobby/scriptaculous