Since the first it born dojo is one of the extjs competitor, one of the advantages of dojo is the license. you can use it for free even for the commercial project, it is created by sitepen, unlike extjs who separated the mobile feature in different framework, dojo include the mobile feature.
Created by Isomorphic Software, smartclient require JDK to work on you machine, smartclient offer variety off license you can get the free version with very limited features, you can see the smartclient version comparison.
ZK framework developed and maintained by Potix Corporation, not only offering framework that ready to use by developer but there also a ready to use application like pivot table and spreadsheet, also there is plugin to integrate to eclipse IDE.
Like smartclient, DHTMLX not completely free, it charge by what the features you want to use, see this licenses comparison.
Wijmo is a kit of web UI based on JQuery UI, is offer many ready to use widget, and it can be themed based on JQuery UI theme, but unlike it core JQuery UI license, wijmo only free for the trial edition.
Vaadin is java web application framework, which mostly used by Java developer, it created by Vaadin Ltd previously named IT Mill.
Echo Web Framework
Not really know about this one, it seems it already long time not being updated, but the web still active and you can still download it.
Some Others Web UI Widget
- Uize by Chris van Rensburg
- Alloy UI by Liferay
- YUI Library by Yahoo!
- Uki Simple UI Kit for complex web apps