The Gmail app is the official client for Google's free, advertising-supported email service. Gmail can be accessed as secure webmail, or via POP3 / IMAP protocols. It offers integration with other Android applications and Contacts. hot
K-9 Mail is an open source email client (a Mail User Agent) for the Android operating system. The application concentrates on being able to handle mailboxes with large volumes of email. hot
Email Me is a handy app which enables you to quickly email yourself (or anyone) notes, web pages, tasks...anything you want. This app creates a pre-populated email using the email address, subject, and message of your choosing.
MailDroid is an ad-supported WebDAV/POP3/IMAP Idle Push mail client written from the ground up and and not based on the stock Android mail client.
Send Mail Assist
Send Mail Assist is a productivity application which lets you create email templates with the minimum fuss and bother.
Yahoo! Mail is the official Yahoo! Mail app for the Android platform.