Mac: We've always looked at email as a stream of messages from newest to oldest, but an app called Unibox decided to take another approach. It sorts your email by person in an effort to make messages more conversational. While it won't work for everyone, it can help those who prefer a more chat-like format for communication.
Unibox feels like a cross between iMessage and Sparrow, our favourite email client for Mac. It still sorts by date, but you get message history with people instead of by subject. That means a recent message from a friend will not only contain that message thread, but many recent threads (if not all) as well. While you can archive mail and interact with it much like you do with a standard client, that's not so much the goal. Instead you just click on the people you want to converse with and send your messages.
This approach won't fix email, much like anything else, but it does provide a much simpler interface for those who don't have mounds of email to get through. Basically, you probably won't want to use Unibox for work, but you might enjoy it on a personal level.
Anyone can set up an account with Unibox and get started easily. It detects settings and requires very little that the user can't intuit themselves. People without the general technical aptitude for email will probably love it, so if you know any Mac users confused by email you should send the link their way.
Unibox ($10.49) [Mac App Store]