One holdout in the push mail space has been Google. Users, especially iPhone users, have requested Gmail and Google Apps mail push support for years, but IMAP (which can be similar, but not as instant) was the best that they could get. But today, Google's Mobile blog has announced that Google Sync now supports push Gmail!
The service is now working for iPhone, Windows Mobile and Symbian S60 devices (BlackBerry users get push mail with their Gmail accounts just by using RIM's BES or BIS system).
If you already have Google mail accounts connected to your phone, chances are you are going to have to delete them and add them again. Google Sync works using Microsoft Exchange's ActiveSync protocol to keep mail, calendars, and contact data in sync.
You have to add your Google account as a Microsoft Exchange account if you want to get push functionality (and only one Microsoft Exchange account per device is supported on iPhone)
For iPhone users, the setup guide is here. Although setting up new accounts for all of my existing mailboxes was a bit frustrating, the payoff of having push mail on my iPhone makes it worthwhile.