Many people believe TrueCrypt is open-source but actually is not because even though the source code is available, it's development is kept secret and it isn't considered Free Software by the FSF nor Open Source by the OSI. A truly open source application for encrypting your partitions is DiskCryptor, but this app only works on Windows.

So if you want an open source application for Linux which works great, you should try ScramDisk.

ScramDisk for Linux (SD4L) is a great OTFE alternative that also supports TrueCrypt containers. Official description: