If you’re a film buff, you absolutely have to stop what you’re doing and go visit this page, a collection of some of the great tracking shots in film. To the ones on that page, I’d add (as others in the comments noted) The Contender and Swingers.
And do yourself a favor: while the tracking shot in Children of Men will likely be considered among the top 3 tracking shots of all time as more people see it, make sure you see it in the context of the film first. And, as the author notes, the shot near the end of the film in the building under siege is equally gripping (slightly less technically sophisticated, but as impressive for different reasons).
(Link was today’s pick at Very Short List. If you’re not subscribed to their e-mail, you’re missing out. I make it a point to read their e-mails every day, the things they find are rarely “popular” yet but almost always fascinating, valuable or entertaining. And many times, they’re all three.)