Most OS X hints are cumbersome and inelegant, but this one rocked my world: Push email in Mail.app
A great way to keep your mailboxes up-to-the-second current without having Mail.app hog the bandwidth
Yes, I hate email as much as anyone else. Nope, this doesn't make sense to me either.
Update (2026): This hint dates from 2008 and applies to old versions of Mail.app; it is unlikely to apply to current versions of macOS.