Zurmo
Zurmo is the best open source CRM software for businesses of all sizes. It sets itself apart from many other CRM solutions on today's market by providing superior contact management, marketing, social, mobile and gamification features. Once you've tried Zurmo, you will quickly see that it's a unique CRM unlike anything else you've seen before. All of the features you could want are there: contact, deal, activity and product management, full customization, internationalization (multiple currency support with instant conversions, different language packs includes, support for multiple time zones), integration with plenty of other online systems and apps, as well as complete "on the go" features allowing you to bring your CRM with you on your iOS or Android mobile device. As Zurmo is an open source product, you won't have any subscription or licensing fees to pay. You get extensive documentation and community support to help you get started.