Gmail - This is app is so good that Google had to go out and buy Sparrow! Sense the sarcasm?
Mail.com - A marketing platform.
Mailbox - Nice user interface.
Yahoo! Mail - Who is using this, seriously?
Windows Live/Hotmail Push - How did I know it would have 1.5 stars?!
Mail Pilot - Mail as a to do list.
Safe Gmail Free - Need to enter password to enter the application, supports multiple accounts.
iMail G - Same as "Safe Gmail Free"
Sparrow - Unified inbox
Handle - Just a to do list for now.
Group Email! Mail - Groups, templates, dropbox, various attachments
Mail 2 Group - Language support, groups and attachments
OMP - Exchange only, meeting requests accept/declines, passcode
Mailshot - Siri support, main focus is to create email groups.
GW Mail - GroupWise email client
Cloze - email and social, contacts whitelist, most email systems.
Good for Enterprise - an app for paranoid IT Managers that want to give Exchange to their iPhone employees without really giving it to them. "Here, use this, its 'good' for Enterprise."
eMailGanizer Pro - for people that use folders and have multiple email accounts, good integration with other apps also
Secure Gmail - More secure Gmail.
Triage: Email First Aid - App for plowing through your email, archive or save for later.
Mail + for Outlook - Outlook email and calendar, some swipe functionality.
Email and Walk - Camera shows what's in front of you in the background while you compose emails.
Better Webmail - Security, multiple accounts, full screen.
Message Finder - Easy searching for messages with several canned searches ie shopping or travel (otherinbox).
Unsubscriber - Easy unsubscribing (otherinbox).