Are you a Beatles or a Stones person? Star Trek or Star Wars? Web mail or desktop client? If you're the sort of person that likes your mail on the desktop, the way Nature intended, then you might want to check out Zimbra Desktop, a free, Open Source, unified mail client for Windows, Mac, and Linux.
Zimbra is a Yahoo product, but it supports far more than just Yahoo Mail -- you can add pretty much anything you've got, from Gmail to Hotmail to AOL, and even POP and IMAP for good measure. Your Gmail labels are represented as folders for consistency with the way other mail is displayed, and you can see your messages as nested conversations. You can sync Zimbra with your Yahoo and Gmail calendars as well.
There's a lot more under the hood. You can create and edit documents in Zimbra and share them in e-mail. If you subscribe to the Zimbra Collaboration suite, you can share docs online without e-mail, making Zimbra sort of like a SharePoint solution for document collaboration.
I've given Zimbra a spin, and while it's nice to get most of my mail under one roof and have access to all my messages -- even Yahoo -- while offline, Zimbra has a very last century feel to me, and the best stuff -- mapping, photo, and local service mashups called Zimlets -- didn't seem any more convenient than what you'd get in your Web browser. You might want to give Zimbra a shot (it is free, after all), but I think Yahoo has a dud on its hands. Disagree? Tell me what I'm missing in the comments.