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Not social, but still sweet - Fluxiom is an asset management system for businesses. It's hard to make "managing your digital assets" fun, but the Fluxiom guys just about get away with it. The blurb:
Fluxiom makes it easy to organize and share your digital assets within your company, your colleagues and friends. Manage any kind of file like corporate media assets, marketing materials, product folders, contracts, images, text documents, logos or artwork – you name it. You don't need to install anything, and there's no IT department required. Plus, it works where ever you are. All you need is a web browser and an Internet connection. Access your digital assets any time you're online.