Veracrypt is an Open Source successor to/fork of the now-defunct TrueCrypt. Its Sourceforge page is the following:
http://sourceforge.net/projects/veracrypt/.
Since TrueCrypt was abandoned by its developers, there has been a pressing need, especially on Windows boxes, for a maintained replacement.
VeraCrypt is developed under the Microsoft Public License, which was chosen for its compatability with the original TrueCrypt license. Linux versions are also available. At present VeraCrypt is not backwards compatible with TrueCrypt 7.1. It is, however, more secure in that it uses much higher iteration counts (one of the principal security audit concerns concerning TrueCrypt) to defend against brute force attacks that technological advances have made far faster and cheaper. A good discussion of VeraCrypt (with several uninformed, ad hominen attacks adroitly defused by a VC developer) can be found at
https://forum.truecrypt.ch/t/why-not-veracrypt/133/10.