Prototype : It is really great way for developing visual effects, animations and user interactions in real time. It’s a light-weight toolkit with faster iterations.
Mootools : Mostly used for creating visual effects – such as sliders for financial websites with stock tickers as an example, slideshows and so on.
Spry : It’s developed by Adobe and used by large corporations rather than smaller in-house companies.
Google Web Toolkit : It’s knows as GWT which talks to Java and used to create Rich AJAX applications. It is an open source framework.