vtiger
From the official vtiger website:
vtiger CRM is 100% Open Source Customer Relationship Management solution built over LAMP/WAMP stack and other third-party open source packages.
The vtiger CRM software installation is hassle free, as we have bundled well-tested third-party software, such as Apache, MySQL, and PHP along with CRM-specific source code. Most of the other open source CRM software providers offer only the application and ask users either to install all other LAMP/WAMP third-party software or approach them for installation.
The vtiger CRM software provides enterprise quality business enhancement utilities, such as vtiger Outlook Plug-in for Microsoft® Outlook® users, vtiger Thunderbird Extension for Thunderbird/Mozilla users, and vtiger Office Plug-in for Microsoft® Office® users. These utilities are part of the vtiger CRM Open Source project in SourceForge.net.
Currently most of the other Open Source CRM projects are not providing add-ons for Thunderbird Extension and Microsoft® Office® and you have to buy the Microsoft® Outlook® plug-in separately.
To learn more about this CRM solution and all the other CRM solutions that Wallanet offers go here: http://www.wallanet.com/open_source_packages/crm