[TrueCrypt is not exactly "Open Source", but the source is available. See the reviews to get more information.]Discontinued
The development of TrueCrypt was ended in 5/2014 after Microsoft terminated support of Windows XP. Windows 8/7/Vista and later offer integrated support for encrypted disks and virtual disk images. Such integrated support is also available on other platforms. You should migrate any data encrypted by TrueCrypt to encrypted disks or virtual disk images supported on your platform.
BitLocker Drive Encryption is a data protection feature that integrates with the operating system and addresses the threats of data theft or exposure from lost, stolen, or inappropriately decommissioned computers.
AES Crypt is an advanced file encryption utility that integrates with the Windows shell or runs from the Linux command prompt to provide a simple, yet powerful, tool for encrypting files using the Advanced Encryption Standard (AES).
DiskCryptor is a free and open-source utility that offers encryption of all disk partitions, including the system partition. Originally conceived as a replacement for DriveCrypt Plus Pack and PGP Whole Disk Encryption (WDE), the project aims to create the best product in its...This app doesn't have any features at the moment.
FreeOTFE is a free, open source, "on-the-fly" (OTFE) disk encryption program for PCs running MS Windows and Windows Mobile PDAs (FreeOTFE4PDA).Discontinued
Discontinued per Wikipedia page (https://en.wikipedia.org/wiki/FreeOTFE)
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...
LibreCrypt (formerly DoxBox) is an open-source disk encryption for Windows and the successor of FreeOTFEThis app doesn't have any features at the moment.Discontinued
Last release Nov 24, 2015https://github.com/t-d-k/LibreCrypt/releases