CIO Insight in it top 10 list of open source mentions Sugar CRM, SplendidCRM, Hipergate and ConcourseSuite as the being the most popular. And of course there is 37Signals product which is offered as a hosted solution.
The full CIO Insight is here. And if you are a microsoft user, Microsoft CRM is a good option.
Also a somewhat dated article for webworkerdaily.
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