Ext3cow is a versioning file system for Linux 2.6.x. “It provides a time-shifting interface that allows a real-time and continuous view of the past. This allows users to access their file system as it appeared at any point in time.”
Main selling point for this seems to be the compatibility with various government data retention requirements, such as FISMA and Sarbanes-Oxley Act.
Read more at The ext3cow File System homepage.