For accessibility reasons, it's best to have high contrast text on a non-patterned background.
However, if you must have a fancy background, or you need a widget to turn off the styling of other people's sites, here's a neat bookmarklet that you can drag to your browser toolbar.
B on W
(via 456 Berea Street)
There are also browser add-ons for accessibility that you can get to help you test the contrast of a page.