From what I have read on the fastmail.fm blog, etc, they tend to do some very good stuff on the back-end, particularly(open source) contributions to the Cyrus IMAP server. These, I understand, have improved the failover and resilience of the IMAP server a lot. These might not be "innovations" in a technical sense, but they certainly make Cyrus a much better server for Enterprise users.

The less said about fastmail.fm's "innovations" on their web interface the better.