Android only: If you don't use Gmail or Google Apps, the default email experience on Android is, honestly, pretty cruddy, unless Motorola or HTC fixed up your Email app. K-9 Mail cleans up, optimises, and just plain fixes Android's default Mail app.
What K-9 Mail does is not all that exciting to describe, unless you've used, or tried to use, Android's Mail app and found it entirely not up to the task. With K-9 Mail, the methods for filing messages, creating email signatures, keeping your mail pushed across all folders and emptying the trash, for goodness' sake, are all available and just better. You can save attachments to your SD card, too, and support for IMAP and Exchange servers is much improved.
K-9 Mail states on its homepage that its mission is to be incorporated into Android's default Mail app. Here's hoping that happens. In the meantime, it's a free download for Android systems only.