Mac: Email can be organised in all kinds of ways, but if you treat your email like a task list, Mail Pilot is a great app for organising it on the iPhone. Now that same basic app is available on your desktop as a free beta.
Mail Pilot works almost identically to a traditional to-do list. When you receive a new email you can take care of it and immediately archive it, leave it around, or set a reminder to take care of it in a couple of days. Likewise, you can also group your emails into lists and set them aside from your current inbox. It's a pretty slick system that works really well on the desktop.
Currently, Mail Pilot only works with the bigger email providers like Gmail, Yahoo and AOL, but it looks like other mail services will be supported in the final release. You can sign up for the public beta below.