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There's an excellent article at A List Apart by Tim Murtaugh called "Mo' Betta Rollovers" which shows how to use CSS to provide a rollover effect.
Tim Murtaugh's Article [alistapart.com]
Arguably, it's a superior method to using javascript because it's faster and it doesn't require javascript support to be enabled in the browser.
An even more powerful CSS-based rollover technique - and the most impressive I've seen - is described by
Petr Stanicek,
on this page [pixy.cz].