We've mentioned Kerio MailServer before, pointing it out as an Exchange alternative. Well, just two days after the release of the 3G iPhone, Kerio announced version 6.5.2 with full iPhone compatibility, complete with push email, contacts and calendar thanks to ActiveSync.
Kerio MailServer has technically supported the iPhone for about a year, using a sync agent that was only capable of synchronization through IMAP, and calendar and contact syncing was handled through iTunes. With newly available technologies, MailServer users have a much more efficient means of keeping everything flowing. It's also been pointed out that, unlike some other email servers, it allows full access to all of your folders, not just the Inbox. According to Kerio, it's real "push," and it really works.
I'm not currently running Kerio MailServer, so I can't attest to its effectiveness, but the 3G update is free for currently active subscribers.
Using it on a previous generation iPhone, however, does require a paid upgrade*. Check Kerio's site for more information on pricing and features.
*Correction: it's a flat out free upgrade for all current subscribers.