One of Apple's oversights is not including a power/resource friendly screen saver. All the included screen-savers are effects-heavy and there is no option for a simple blank screen as there is in Windows. Indeed, you could say that their position of being a 'green' company could be greatly enhanced by this one simple thing. There have been solutions to this in the past, however with Leopard and Snow Leopard many of these things broke. A guy called "Dr. Lex" (né Alexander Thomas) has a solution.

The most effective screen saver you've ever seen. Guess what: it just makes the screen turn black! Great if you don't want your screen to burn in neither waste precious processor cycles with some fancy animation which distracts you from your work. Now compatible with Snow Leopard so you can harness the power of 64-bit processing to display absolutely nothing on your screen.

Full link: http://www.dr-lex.be/software/screensavers.html