Boxcryptor is an easy-to-use encryption app optimized for the cloud. It allows the secure use of cloud storage services without sacrificing comfort. Boxcryptor supports all major cloud storage providers (such as Dropbox, Google Drive, Microsoft OneDrive, SugarSync) and supports all the clouds that use the WebDAV standard (such as Cubby, Strato HiDrive, and ownCloud). With Boxcryptor your files go protected to your cloud provider and you can enjoy peace of mind knowing that your information cannot fall into the wrong hands.
While the basic plan is free, there are also personal and business editions (per year). Click here for full pricing info.
Boxcryptor 1 is now a separate fork, called Boxcryptor Classic, which can be found in "Related Links" below.