NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
Roundcube - Web-based IMAP email client
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.