Just learned about Bitmessage. I suggest people go read the white paper on their wiki - short and to the point. It has the potential to upset email because of the increased privacy, built-in mailing-list functionality, and anti-spam measures (proof of work makes it hard to create lots of messages fast). This Open Transaction use case is a perfect example of the innovation possible with it.
It's not going to replace email. I've been looking at the code and it's not quite the work of art yet but it's more a hobby project. That said it fills an important niche at an important time.