Two classes: Ext.ux.WebWorker and Ext.ux.WebWorkerManager.
The first one is a wrapper for standard HTML5 WebWorker and it provides a lot of interesting and easy-to-use features. The second one is a singleton to register different Ext.ux.WebWorker and it provides functions to work with every registered webworker at the same time.
So, with this extension, you can easily use Web Workers for your applications!
Read the README.md file on Github, follow the example and start playing with it!