Everyone knows there has to be a better hub for your mail than the current Exchange, but you can't adopt a system like Zimbra or Open Exchange without making changes to all your desktop
and mobile clients. (corrected- Gray pointed out that ActiveSync means no mobile headache!) So I was excited to see a server solution that aims to leave the rest of your communications world untouched, PostPath. They emulate a whole series of Microsoft's proprietary communication APIs, and they've done it the hard way, sniffing network packets since they were tackling this before the latest Open Specification releases.
They seem to have done an impressive job emulating Exchange, with a good crop of real-world deployments showing that it's at production quality. There's some things I'd like to figure out, like how much of Exchange you need to keep around for Active Directory and management tools, but I'm looking forward to downloading the evaluation version and seeing for myself.