With a name like SystemRescueCD, there are no prizes for guessing what this does. This is a mini-Linux distribution on a CD that can be used to recover or repair a broken system. Although it is a Linux system, it can be used to repair Windows computers too and includes a number of tools specifically for this purpose.
The name is slightly misleading as it can also be copied to a USB flash drive, no self-respecting sysadmin should be without one of these in his pocket at all times. It can even used to boot a broken computer over the network using the standard PXE network boot protocol, so even a broken computer with no CD drive and broken or disabled USB ports can be fixed.
There are two modes of operation, SystemRescueCD can boot to a lightweight graphical desktop, which is ideal for those less familiar with Linux and its command line tools. From here, any reasonably experienced admin, whatever operating system they normally use, should be able to find their way around. For the hard core Linux user, you can boot to a text console. For those used to the command line this is not only a faster way of working, it boots more quickly too. Having broken a number of systems over the years, usually with some provocative maintenance, I have yet to find a breaking this can't be used to fix.
No self-respecting sysadmin should be without this.