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Just be cautious of 'open' software, regardless of who is behind it, as free is not necessarily better when it comes to security of a network. Additionally, this type of software requires experience load and customize, which means you will be paying for it eventually.
This all depends on need, budget, abilities, security policies, and time.
What do you need it for...data storage and backup, document uploads and management, online customer support (appointment setting, etc.), online payments, application use?
If it is simply for data storage, the easiest option may be for you to utilize an existing service, such as Amazon's AWS. May I refer you to: http://aws.amazon.com/products/
If you want to provide applications via cloud, then Google Apps, or Microsoft Office 365 may be an option. PC World has a good article on comparison of: Microsoft Office 365 vs. Google Apps
If you just want, or need to keep it in-house, it is not complicaed. Hardware will consist nothing more than server-based equipment.
Software...though it's a bit technical, you could always go the virtualization route with a software package such as VMWare's EXSI. This would allow you to customize and grow your storage as needed very easily.
I also read a good article on Inc. last year that overviewed a number of tools...do a search for 'Cloud based software'
As a reference, you may also want to read the following article: What is Cloud Computing