Not an easy answer! I would say have a look at Vtiger (been around for quite a while now), ZohoCRM (it’s not free for more than 3 users I think) and SugarCRM (quite popular and mature).
There are loads of open source CRM applications. Some of them are written in PHP some in Java and some in other languages so have a look and try to install some of them. If you don’t have web space to try them, leave a note and I will try to help.