I know a lot of people who stick with their webmail interface because it’s simple and does everything they need it to do. The nice thing about Inbox2 is that it may be a desktop mail client, but it tries to aggregate mail from all of your different accounts as well as your social networks. It supports Gmail, Yahoo, AOL, Exchange, Facebook, Hyves, Hotmail, Twitter, Yammer, LinkedIn, and many other mail accounts that can be set up manually.
If you’ve used Gmail before you should definitely feel comfortable using Inbox2, because a lot of the interface appears to be modeled after it. The other thing that Inbox2 offers is a combined list of all your contacts across all your mail and social accounts, which helps find that person you need to send a message without having to think about where their contact information is located.
The people behind Inbox2 have also made a free iPad app that they originally called Inbox2. They have since renamed it to Fileboard, but it remains free. So if you enjoy the desktop app it may be worth checking out the iPad version as well (though it doesn’t bear much resemblance).
Inbox2 Homepage (Windows only; Freeware)