I've just played a little with the last release of Zoë. It's an amazing example of what you can achieve using open standards.

Just to give a few examples: Zoë is a mail management tool, it's web based, so you use HTTP to control it, and it sychronizes with your email client using POP, IMAP and SMTP. It incorporates an FTP server, so you can connect to the running tool and mount a volume containing your email attachments. It also creates an RSS feed of your mail and, with this last release, it includes an RSS aggregator. The tools also supports Blogger and MetaWeblog API, so you can interact with it with tool such as Radio UserLand or NetNewsWire. Of course, all this stuff can get back to your email client, and you can post to your weblog email messages trough the RSS feed...

I still have to figure out what else can be done with it, anyway it's definetly a very interesting approach.