Andy posted some jQuery that showed the user ids on the site admin users screen. I wrapped the jQuery in a plugin which you drop into your mu-plugins folder.
Show user id in site admin
August 23, 2009 By
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simple wrapper on a simple line of js!
you can make it a few bytes smaller by removing the var user_id = from the start of the line, that’s only there to stop the page from changing if you paste the line into the address bar of the browser.