K-9 Mail is an open-source e-mail client with search, IMAP push email, multi-folder sync, flagging, filing, signatures, bcc-self, PGP, mail on SD & more! It is based on the Email application shipped with the initial release of Android. K-9 supports IMAP, POP3 and Exchange 2003/2007 (with WebDAV). With over 800,000 downloads and 350,000 active users, K-9 mail has a large, enthusiastic installbase. It can be obtained from the Android Market here: https://market.android.com/details?id=com.fsck.k9&feature=search_result

Check out our ideas page for GSoC. If there's an idea you're excited about, please discuss it with us at http://groups.google.com/group/k-9-gsoc