Originally taken from http://extjs.com/learn/Manual:Resources#Aptana_.2B_Spket_.2B_ExtJS

1. Download and install Aptana Studio (includes Eclipse).
2. Download the Aptana Ext JS 2.0 plugin
3. Start Aptana and navigate the application menu to: Help → Software Updates → Find and Install... → Search for new features to install → New remote site...
4. Name: "Spket", Url: "http://www.spket.com/update/"
5. Restart Aptana
6. Watch this Spket IDE Tutorial to see how to easily add Ext code assist (you can point it at the latest /src/ext.jsb to keep code assist up to date with the latest Ext version). The steps are basically:
   1. Window → Preferences → Spket → JavaScript Profiles → New
   2. Enter "ExtJS" and click OK
   3. Select "ExtJS" and click "Add Library", then choose "ExtJS" from the dropdown
   4. Select "ExtJS" and click "Add File", then choose the "ext.jsb" file in your "./ext-2.x/source" directory
   5. # Set the new ExtJS profile as the default by selecting it an clicking the "Default" button on the right-hand side of the "JavaScript Profiles" dialog.
   6. Restart Aptana
   7. Create a new JS file and type: Ext. and you should get the Ext Code completion options.

NOTE: On Ubuntu x86-64, I had to get eclipse, install Aptana a plugin, and then go from there.