The default email application on the iPhone is decent, but the one thing it lacks is landscape mode. Wide email allows just that and a nice feature set for $1.99.
• Use wider, horizontal "landscape" keyboard to write emails
• Works for replying to emails from Mail, including reply all (see below)
• Turn phone to any orientation (even upside down!) and interface rotates
• Save drafts to finish later — great for writing long emails to friends
• Automatically saves copies of sent emails, just in case
• Automatically saves drafts if program is interrupted, e.g. by home key