Run your own mailserver. If you don't trust Google with your data, there's nobody else I would trust. There are plenty of open-source webmail apps if that matters to you.
Setting up your own mailserver is easy. But getting a gmail-like experience is pretty hard. For example, I tried Roundcube a while back with my full Gmail mail dump. It tried to keep up, but there was just no way it was going to handle 2gb of mail going back years.
I'd love to see something that either ran off of a standard imap server or that was a mail-delivery agent (MDA) that had a gmail-like web interface, complete with full text search.
I haven't found anything like that yet.