OX Documents: Online-Office-Suite als Open Source - Golem.de. Another candidate worth taking a look at - especially since the OX App Suite can almost completely replace Google Apps. However, I would then have to handle spam filtering myself (or simply route it through Google and have it sent directly to me, as there was an option for that). It somewhat conflicts with what Owncloud does, so I'll have to see which of the two services is actually more interesting for me. But maybe file syncing will also come to OX App Suite soon.