PeoplePods is a free social networking twitter clone software to create websites with social networking capabilities easily. This twitter clone can be integrated into an existing site or it can be used as a standalone website. Simple to use, but I think some design work is needed for this free twitter clone .
It has the usual twittter options like voting, bookmarking, groups etc.
Price: Open source
Demo: core.peoplepods.net
More info and download: peoplepods.net
Follow Me