Thank you for the concrete suggestion! From the screenshots, Roundcube looks like a very good webmail client. It looks like they're going after Outlook though with their feature set and layout.

[Edit] But this is exactly what I'm looking for in terms of being an installable package that can be thrown on a web server and hooked up to an in-house or third party email infrastructure.

Roundcube is good and has come a long way over the years.