- fakeroot: Gives the running program the impression that it is running as root, often used for example for building debian packages. This will let the build script create a directory tree which it believes is owned by root and then when that directory tree is packed by tar, tar will also see root as the owner and the .tar-archive will have root as the user/group for the files in the final debian package.
- faketime: Gives the running program the impression that it is running at some specific time. Usefull for testing code during specific events like leap-years, etc.
- eatmydata: Will ignore all fsync() and related system calls which ensures files are written to permanent storage. I have used this once for running a database during a testsuite, and databases are much faster when they do not have to wait for the data to reach permanent storage.