Petr Stanicek has devised a clever technique to do image rollovers without resorting to JavaScript and with no need for preloading. Check out the examples to see if you can figure it out for yourself before reading his explanation - that "eureka!" feeling would be that much more gratifying.

For the eternally constipated, zoom over to "Fast rollovers, no preload needed" for the explanation. (via Simon Willison's Weblog)