Mail2web
Mail2Wap allows you to anonymously pick up your e-mail from any POP3 server

Send E-Mail
Send E-Mail from Wap

Gmail Mobile
Gmail Mobile

Windows Live Mail
Check your Hotmail email on your mobile phone.

Gala.net Mail
Free

Gmail
Googles Gmail for mobile devices.

MailMate
Free email service

Mobulus - Mobile Email
Access your pop3 imap4 emails on any mobile phones

Quick Mail
QuickMail lets you send anyone an email from 8 preset phrases such as, Whats Up? It sets this text as the subject of the message.

Wide Mail Keyboard
Lets you write e-mails using a large keyboard while holding your device in landscape orientation which isnt currently supported by Mail on iPhone

iVoiceMail
A one night hack to get GrandCentral Visual Voicemail on the iPhone. Allows playing and archiving all new messages. WebAppUI from apple used for style