That would be a nice app to have for sure. I've been looking and haven't found one, but you can hold alt and press the back key (the curvy arrow next to the trackball) and t will show you everything that's open. You then have to manually close them.
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Are you referring to a personal task manager or BB OS task manager?
Best To-Do app I have played with so far is ToDoMatirix but I am not completely satisfied with it.
If you mean BB OS task manager, then I dont know that it would actually make your life easier as it would just be another app that is open all the time...
Push the left conv button, between the USB and MicroSD card slots. Or push Alt-Back key. A window will open up and use the trackball to move left and right to see everything that's running.