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John Resig (of jQuery fame) has a bunch of interesting examples of using web workers here – games, graphics, crypto.
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Another use is Web I/O – in other words, polling URLs in background. That way you don’t block the UI waiting for polling results.
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Another practical use: in Bespin, they’re using Web Workers to do the syntax highlighting, which you wouldn’t want to block your code editing whilst you’re using the app.
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From Mozilla: One way workers are useful is to allow your code to perform processor-intensive calculations without blocking the user interface thread.
As a practical example, think of an app which has a large table of #s (this is real world, BTW – taken from an app I programmed ~2 years ago). You can change one # in a table via input field and a bunch of other numbers in different columns get re-computed in a fairly intensive process.
The old workflow was: Change the #. Go get coffee while JavaScript crunches through changes to other numbers and the web page is unresponsive for 3 minutes – after I optimized it to hell and back. Get Back with coffee. Change a second #. Repeat many times. Click SAVE button.
The new workflow with the workers could be: Change the #. Get a status message that something is being recomputed but you can change other #s. Change more #s. When done changing, wait till status changes to “all calculations complete, you can now review the final #s and save”.