This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
See the comparison matrix in the references section of the above URL.
Different implementations provide rich component libraries. Also there is strong AJAX support in many of the frameworks. Though the portability is still not good between frameworks but hopefully that will improve with time.