Tuffmail offer an exceptionally advanced mail hosting service. The feature list is very detailed and very technical. All accounts are monthly bandwidth limited (related to the disk limit). Starts from 1 account at $2.40/month.
Lists webmail providers including a list comparing features such as own domain, imap, pop3, etc. Makes a useful list when looking for an outsourced mail provider.
FuseMail is a hosted mail service which includes webmail, IMAP, POP3, calendar functions, shared files, groupware, and more. All for $2/month/mailbox. Great.
Google Apps is the hosted mail / docs / etc product from Google. It comes in Free or Premier ($50/year/user).
Zoho is an online office suite, similar to Google Apps. It includes a hosted mail product.
Zimbra is an open source email / collaboration software. They offer a hosted service starting from $400/year for 15 pro mailboxes.