Sookasa!

[TrueCrypt is not exactly "Open Source", but the source is available. See the reviews to get more information.]

Cryptomator provides transparent, client-side encryption for your cloud. Protect your documents from unauthorized access. Cryptomator is free and open source software, so you can rest assured there are no backdoors.

Boxcryptor is an easy-to-use encryption software 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. »

1. A Real Drive. Creates a virtual hard drive that is indistinguishable from a real drive. Stores the actual data in the cloud provider of your choice (or locally).

A program to mount EncFS folders on Windows and OS X. Works good with LibreOffice documents on Windows 7.

Securely encrypt your private and confidential files or whole folders.The encryption/decryption process is simply done by dragging the selected file(s) or folders(s) to application window.Files are encrypted using strong encryption algorithm: AES (Rijndael) 256bit, RC6 256bit, Serpent 256bit, Blowfish 448bit, Twofish 256bit, GOST 256bit + (Threefish 1024bit and SHACAL-2 512bit for Pro Version) ciphers are available.Cross-Platform (Windows, Linux, Mac OS X, ...) GUI Version and Command Line Version are available.Compatible with the Secret Space Encryptor for Android.A wide range of compatible platforms (desktop and mobile) allow easy sharing of protected data.Easy to use. »

CryFS encrypts your files, so you can safely store them anywhere. It works well together with cloud services like Dropbox, iCloud, OneDrive and others. The goal of CryFS is not only to keep file contents, but also file sizes, metadata and directory structure confidential.

All data is encrypted before it leaves the computer of the user. Encrypted data can be shared with other users. The encryption and all necessary keys are under the full control of the user. PanBox was developed for Windows, Linux and for Android-based mobile devices. The application is optimized for usage with Dropbox but can also be used with other cloud-storage services or file shares.

Perfecto Encryptor will encrypt your data fast and securely!

Safely encrypt sensitive dataOn PC, in networks and in the Cloud

The software Cloudevo aggregates any number of cloud storage to a big united cloud. It is unessential from which provider you have your cloud storage. Cloudevo combines all of the different cloud services to one large cloud. This united Cloud is then mounted as a drive on your device. All files are encrypted securely on the Cloud drive. In addition to encryption, the software includes useful functions, which facilitate the handling, administration, and synchronization of the data in the Cloud. You can access your files in Cloud Drive from anywhere. »

Data encryption software by CryptoForge allows you to secure your sensitive data everywhere it goes with strong AES encryption. It features a file encryption module which allows you to encrypt files and folders of any type and size, on any kind of media, including external USB drives, mobile devices, flash memories, and network folders. It also includes a file shredder for secure data disposal. Email encryption is also very simple since recipients of encrypted files don't have to install software to be able to decrypt the files. The Text encryption module converts anything into ciphertext. You can send encrypted text via any messenger program, or even post the ciphertext to social networks. This encryption app also features multiple encryption, file name encryption, data compression, cipher speed meter, four encryption algorithms, and a complete command line interface for easy integration into existing systems. »