A design lens allows you to view your user experience design from the perspective of a single design principle. Lenses were originally created for game design but are just as powerful for user experience design.
I recently started documenting lenses that I find useful on a new site Designing with Lenses. Though it only has 3 lenses to start with, more lenses are coming in the months ahead.
To get the whole scoop, go read the introductory article Designing with Lenses over on UX Booth.
This article is part of a series of posts written by the speakers of UX Lx, The premier UX conference in Lisbon, Portugal; happening on May 12th–14th. There's a great lineup of speakers and Portugal is absolutely beautiful, so head over to the conference site. You can enter UXBOOTH2010 for a 10% discount.
To get updates on new lenses follow my uxlenses twitter feed here.
I will also be talking on this subject at CHIFOO in Portland tomorrow night (April 7 at 7pm). Join me if you can!