Pipes - I install it on every device I've used for a long time, it's a fantastic time waster when you're waiting for something, easy to put down and pick back up. The grid size is completely adjustable and in fact works very sweetly with the 8100+ trackball, a perfect fit.
My only wish is that it had some sort of system where it remembered how fast you finished a certain grid size (like a top 10 scores) so you can try and beat your best time. I think I'll send that suggestion to the author...
web:
http://kornhornio.net/pipes.php
ota:
http://mobile.kornhornio.net/