Tomb is described as '100% free and open source system for file encryption on GNU/Linux, facilitating the backup of secret files. Tomb is written in code that is easy to review and links commonly shared components'. There are four alternatives to Tomb for Linux, Mac, Windows and Online / Web-based. The best alternative is Kryptor, which is both free and Open Source. Other great apps like Tomb are Hat.sh (Free, Open Source), Age (Free, Open Source) and Cryptmount (Free, Open Source).
Cryptmount is a powerful utility that allows any user to access encrypted filesystems on-demand under GNU/Linux systems without requiring root privileges.It requires Linux 2.6 or higher. It handles both encrypted partitions as well as encrypted files.