In ClearCase, if I am working on some file X and want to also see its previous version (say version 5), it is available as X@@/main/5. Is something similar available with other (preferably free) version control systems?
Mercurial has hg cat and hg co, but they still do not come close to ClearCase's feature above.
A quick Google search found gitfs:
gitfs is a FUSE-based filesystem for working with source trees stored in git repositories. The eventual goal is to provide a convenient way to work with lots of branches and patches. Currently only very basic functionality is implemented -- read-only access to the existing tags and objects.
There are likely other (more active) projects offering similar features (both for Git and other VCSes).