Kaiten Mail - A K-9 variant with a redesigned UI designed to work better on tablets.
vf-pamc - "A K9mail based android email client for the vodafone 360 mail service"
Mail'd for Notion Ink Adam
K9 for Pure widget - A fork of K-9 for the proprietary "Pure Messaging Widget"
Secure9 Mail - "Secure9 Mail is an email client built on K9 Mail v2.9. It adds 128 and 256 bit AES encryption, as well as key management and key agreement through the Diffie-Hellman protocol."
Multimedia Email Client - "Multimedia Email is an email client. [...] It supports multimedia. It can use phone camera to take an image/video as an attachment."; displays advertisements
ECSMail for Android - ECSMail is an implementation of Envelope-Content Splitting for the Android platform and is one of several ECS-enabling extensions offered by ChiaraMail Corp.