One extra vote for Veracrypt (https://www.veracrypt.fr https://www.veracrypt.fr). It's safe, has passed security audit, gets updated frequently, multi platform

One extra vote for Veracrypt (https://www.veracrypt.fr https://www.veracrypt.fr). It's safe, has passed security audit, gets updated frequently, multi platform support with downloads for all OSes and it's free. Although it supports whole disk encryption, it's more suited for a virtual encrypted disk within a file.