Teambox is a neat online project management app that integrates what works with social networking to try to make a more enjoyable and effective collaborative experience. Plus it's open source, so you can fully customise it, too.
While the idea of bringing a Twitter-like experience into a working environment doesn't really sound too appealing at first, it's actually a more speed-appropriate channel for communication (which we like). It's good for reducing email volume, plus project communication is heavily status updates anyhow. While I found the idea off-putting at first, it really looks to be an effective means of communication in the workplace.
Teambox works a lot like most project management software, but with a better flow of communication and overall organisation. While that's the main draw, there are a couple of notable features. Projects can have permissions, so you don't have to give everyone access to the project. Teambox also works a a mobile app so you can use your smartphone for updates when you're away from the computer.
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Teambox provides three projects and 50MB of storage for free, but pay accounts can bring you up to 2000 projects and 250GB (so there's room to grow if you need it). Because Teambox is an open-source project, you can also install it on your own server for free for no limits and full customisation.
If you're looking for something a little more personal, be sure to check out the hive five best project management tools.