While it doesn’t qualify as an open source solution, IBM’s Lotus Notes/Domino gets some space today because of a lively discussion over at Slashdot concerning the products push against Microsoft. Recently announced Domino Express Licensing and Notes 8 places the price at $100 per user. That may still a hefty price for many small businesses, but for some companies that require a robust commercial messaging solution and with the budget to match may find something in it for them.

From the IBM information page:

IBM Lotus Domino Messaging Express software combines full support for
the latest Internet mail standards with the industry-leading messaging,
calendar and scheduling, discussion database and reference database
capabilities of Lotus Domino software - all in one manageable and
reliable package.

Lotus Domino Messaging Express includes a restricted license of Lotus
Domino Messaging Server that removes selected capabilities designed for
larger enterprises. You are entitled to use it on as many machines
within your company as you would like, but access is limited to those
users for which you have paid the per user charge.

ComputerWorld has an article that looks at the new Notes 8.