Noupe.com has posted a top ten list of what they consider some of the best PHP/Ajax-based webmail clients (as based on the user interface and ease of use).
Employees need to access their email from wherever they happen to be â€“ on the road, at customer sites, remote offices, and at home. [...] Today i wanted to share with you 10 AJAX-based webmail client that delivers the look and feel, usability and performance of a desktop application. Some of the webmail clients listed below are open source code that can be easily modified to better suit userâ€™s needs, while others are not. So You must check their license to know the rules.
Included in their list is software like:
Some of these tools offer more than just email too - they can include things like collaboration tools and mobile versions right out of the box.