encfs4win!

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

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. »

LibreCrypt (formerly DoxBox) is an open-source disk encryption for Windows and the successor of FreeOTFE

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

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.

DCrypt is an encryption and decryption tool that combines in a smart way multiple algorithms. The combination of multiple strong algorithms makes it much more difficult for hackers to get the plain content of the encrypted file without the password.

LUKSUS is a tool that creates an encrypted volume and filesystem on a hardrive or other storage media as well as a filecontainer.It can use the following encryption facilities: LUKS, Truecrypt and GELI.

SecurStick is a free portable application that lets you carry encrypted files on your USB drive. You can use the drive on any computer to access and modify those files WITHOUT needing any administrator privileges.

ProxyCrypt is a command line tool that creates encrypted volumes within a file or a hard drive.It works as a "proxy" for the ImDisk Virtual Disk Driver .

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. »

The Gostcrypt project has been launched at the end of 2013 as fork of the (late) Truecrypt project.

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.