Cypht is an awesome Free/Libre/Open Source webmail aggregator written in PHP and JavaScript. Tiki is the Free/Libre/Open Source Web Application with the most built-in features, obviously including a webmail. Cypht within Tiki expands on the collaboration possibilities.

Email handling (reading, replying and archiving) should be part of a larger collaborative workflow. Instead of one gigantic mail store, we should have a number of smaller ones that make sense to one's workflow (ex.: around projects, tasks, clients, etc.) and that can easily be shared and prioritized.