Cryptomator!

VeraCrypt is a free disk encryption software brought to you by IDRIX and that is based on TrueCrypt .It adds enhanced security to the algorithms used for system and partitions encryption making it immune to new developments in brute-force attacks.For example, when the system partition is encrypted, TrueCrypt uses PBKDF2-RIPEMD160 with 1000 iterations whereas in VeraCrypt we use 327670. And for standard containers and other partitions, TrueCrypt uses at most 2000 iterations but VeraCrypt uses 655340 for RIPEMD160 and 500000 iterations for SHA-2 and Whirlpool. »

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

Duplicati is the only portable backup program, able to send backups in encrypted form to another computer or SkyDrive using one of several methods (WebDAV, FTP, SFTP/SCP/SSH, Amazon S3, Rackspace CloudFiles, USB disk, Windows network share). Time scheduler e.g. for auto-backups every 3rd Monday on 10 a.m.German, Danish, English, French, Portugese. Can also be startet from command-line. Support for snapshots via VSS and LVM: Backup of open files. Duplicati is a rewrite of Duplicity/ftplicity, written for Windows (in managed C# .NET). It also runs on Linux using the Mono framework.The lastest portable version in all 5 languages can be downloaded as a zip file from https://github.com/duplicati/duplicati/releases »

Syncany is an open-source cloud storage and filesharing application. It allows users to backup and share certain folders of their workstations using any kind of storage, e.g. FTP, Amazon S3 or Google Storage.

CryptSync is a small utility that synchronizes two folders while encrypting the contents in one folder. That means one of the two folders has all files unencrypted (the files you work with) and the other folder has all the files encrypted.

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. »

Safe is an application that makes it easy to encrypt your files. When you encrypt your files with Safe they are rendered unreadable to anyone who doesn't have your password.

Sookasa transparently protects files across the Dropbox cloud and linked mobile devices, while preserving the native Dropbox user experience on Windows, Mac, iOS and Android.

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

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. »

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