eGroupware is a Web/PHP/MySQL application.
I'ts major force is that it's a Groupware product that has:
Calendar that can be shared with other users
Combined with a imap/pop mailserver this is a really great alternative for Microsoft Exchange (and it's free).
Now; these can actually be syncronized with a client like Evolution (calendar doesn't work), Mozilla Thunderbird with Lightning addon (contacts can be really tricky to sync).
These 2 clients I've tested myself, but there are more.
It has a great Project management with GANT diagrams and billing system.
Management to save your documents so you can access them from anywhere and not trust a third party to have these stored by them.
A great Knowledgebase
Resource management like meeting rooms etc.
there is plenty more, but I'll finish off with telling that different applications can be set to be accessed or denied using a nice user management system that can grant/deny a single user or a group.
at some points it can also replace the Microsoft Sharepoint, of course depending on how you are going to use it