TrueCrypt is an open source software to encrypt volumes of data. It even supports the creation of virtual encrypted volumes which is portable.

DiskCryptor is a free and open-source full disk encryption system.

BitLocker is a full disk encryption feature included with Windows Vista and later. It is designed to protect data by providing encryption for entire volumes. …

VeraCrypt is an open-source utility used for on-the-fly encryption (OTFE). It can create a virtual encrypted disk within a file or encrypt a partition.

CipherShed is a planned source-available disk encryption program for on-the-fly encryption.