I like Martin Fields. It's hard, and thus relatively easy to apply and highly durable, yet doesn't impact the sensitivity (at least on my 5800, my N900 screen protector won't get here until tomorrow) of the touch screen.

Best of all, it has zero impact on the optical quality of the screen.

Expensive, though. Supposedly there are less expensive knockoffs now that use the same material.

Edited to add: I had never noticed much in the way of scratches from my keys/change and stuff on the one on my E71, but now that I examine it closely, it does have quite a few nearly invisible fine scratches that you can only see with the screen off and light hitting it at a certain angle.

It did spend over a year in my pocket, though.