This is not a tip about a specific feature of Mac OS X or of Mac apps, but it looks cool. If you change your desktop background every few weeks, you'll go through the ones included with OS X pretty quickly. This is the website of a freelance photographer who posts many great photos as high-quality backgrounds: http://www.wallpapers.anuli.co.uk/. At full-res, the wallpapers are 1920x1200 (enough resolution for a 23/24" display). They are also available in a full-screen aspect ratio.