Inout Webmail - Inout Webmail script is loaded with state of the art features and it allows all mail operations under the same page.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Posteo - Web- and IMAP-based email service. 2GB cost 1€/month.
HEY - Email at its best, new from Basecamp.
Zoho Mail - Zoho Mail is a secure, encrypted, and enterprise-ready email solution, a suite of apps tailor-made for your organization's needs.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.