Spideroak
Spideroak much like other cloud storage services started as a personal service and grew into an enterprise offering. One of its key selling points is that, unlike some of the other services available, encryption keys are not stored with the service. Spideroak calls this its 'Zero Knowledge' policy and it effectively means that only you have the keys to the kingdom. If your keys are lost, you lose your data, as not even Spideroak can recover it.
This has obvious advantages in the sense that, while it's tempting to entrust your business data to a cloud, ultimately you rely on the service provider to ensure your data is safe -- be it from from hackers, cloud service employees abusing their privileges, or more obtuse threats like the government of the country where the servers reside demanding access.
On the collaborative side, beyond allowing users to sync and share files across devices, Spideroak has 'Sharerooms'. This allows users or administrators to create shared folders that draw on multiple locations across different devices or storage mediums, and making this singular share available to others -- be it internal Spideroak users or external clients -- via a web interface with a login for the Shareroom. This makes it easy to set up, for example, Sharerooms for different departments such as marketing or sales, in addition to employees having their own private shares. As keys are never stored with Spideroak, and while editing files on shared resources is easy to do, keep in mind the Spideroak client will first download and then decrypt the file for editing, which may make working on very large files while on the road problematic (decent internet access not with standing). As always, security is a dance of trade-offs.
A dedicated administration console provides an overview of all shared data in the organisation, as well as enabling tasks such as adding or removing users and groups, manage group shares, view reports, and monitor user activity. File versioning is supported, and taking advantage of its roots as a cloud storage backup service endpoint devices and even key directories on local machines (including the desktop) can be backed up to the cloud.
Sitting somewhere between ownCloud and typical cloud sync and share services, Spideroak can be used traditionally as a cloud service at the enterprise level, or optionally installed within your network, giving you full control over not only access and permissions for data, but how and where the data itself is stored.
Windows, MacOS X, and Linux are directly supported through desktop applications as well as apps for iOS and Android.
Plans for Business begin with a minimum of 10 users, and Enterprise a minimum of 100 users. Regardless the cost is $5 per user per month with unlimited storage, endpoint devices, and versioned files. Aside from the minimum user requirement, the Enterprise version differs from Business in its support for Active Directory and LDAP.
5. ownCloud