DOS File substitution
This is a very specialistic tool, designed to work only in DOS/DOSBox. It intercepts requests to open a file and reroutes them as specified. For instance, your game might try to get its configuration from a file called RESOURCE.CFG, but you want it to read from MT32.CFG instead. If so, just run the command "fsubst resource.cfg mt32.cfg" before starting your game. I wrote this program for just that purpose: to switch configuration files for games stored on a read-only medium.