How do I "End Task" or kill a running app process?

You can do that using any number of 3rd party apps, or by using ASTRO. Follow these steps to place a shortcut on your Home Screen.

Open ASTRO > Preferences (Prefs icon on toolbar) > Home Screen Icons > check the box for Processes

Now tap Home and tap on your new shortcut to open the app. Tap Menu and Kill all to close all apps left running. This frees up system resources so that your apps run more smoothly.

Note: Stick to the Apps tab and leave the Services and Processes tabs alone to avoid system instability.