Relenta (www.relenta.com) makes emphasis on Email being at the center of it all, which is a valid angle. They make good use of groups to enable distributions (email campaigns) and nailed down the concept of departments pretty well with the ability to share contacts and calendars. Relenta email works by connecting to the existing mailboxes, which is the way to do it. Design is pretty good and makes fairly good use of layers to avoid reloads (although could use more). Calendar items currently have to be linked to the contact file (which is limiting) and I didn't see support for iCal subscriptions, which I think is a must for any web-based shared calendar these days.
To round this up, with the addition of web forms to digest prospect data from a web site contact form (which they said they would have in under 2 weeks), this is definitely a viable contender for the Office 2.0 app.
I'm going to give Relenta 4 stars as they are on the right track.
Update: As of late November 2007, Relenta added a web form post mechanism, making it possible to collect customer lead data from the web site. This is a significant improvement in making Relenta a serious contender in the lead management/email marketing space.