Unable to find a ComboBox component for Ext JS that would work in an Ext Grid, permit multiple selections, and support both local and remote datasources with multiple fields (one for display, one for the index), I set out to build my own some time back.
Note: Since I began working on this some time back, a similar Ext plugin was developed by another developer: see http://lovcombo.extjs.eu/
Examples after the fold.