Two years ago, I surveyed the state of GWT widgets. It was pretty fragmented and a bit confusing. While I haven’t been paying very close attention, I don’t think things have changed much.
I was browsing around the GWT google group, and found this interesting announcement.
I’m a huge fan of GWT and the only problem I have with it is not really a problem with GWT but with the fact that there isn’t a centralized place for registering component and associated tools AFAIK. I have created an application to do this which is available at http://gwtmarketplace.appspot.com/
I visited the marketplace and while it is currently in its infancy, it could be very useful. If you have GWT code that you want to share with the world, consider registering it. I added gwt-crypto, which I help maintain.