The best replacement for extjs is jquery.

The best alternatives to extjs are rated as following:
★★★★★  jquery
★★★★  dojo
★★★  qooxdoo
★★★  gwt
★★  dthmlx
★★  yui
★★  jquery ui
★★  angular
★  backbone
★  kendo
★  flex
★  gxt
★  emberjs
★  ext gwt
★  jsf
★  sencha
★  extgwt
★  dhtmlx
  telerik
  ext.net
  smartclient
  smart client
  mootools
  twitter bootstrap
  prototype
  ext core

Your alternative for extjs is not listed? Suggest!
Speaking about extjs, one probably shouldn't forget about grails.

Other search terms related to extjs and their ratings are:
★★★★★ grails
★★★★ apex
★★★★ spring mvc
★ store
You have in mind a companion for extjs and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of extjs in all years. Click on graph for more.
extjs
jquery
dojo