You CAN use another computer, like a laptop. Do you want a GUI?
If not, you can just SSH into the machines. If so, you have to forward X Windows and allow Remote X Login.
That being said, this will only work if your network interface is up, SSHd is running, and the system is stable.
Another option for using a laptop is to use a serial console. There is a great HOWTO
Here. However, this requires your server machine to have the capability (usually in the BIOS) to allow a serial console as the system console (tty0). With a server (purpose-built, like a Dell or Compaq/HP) that shouldn't be a problem, but it might be with a machine that began life as a regular desktop box.
But... why do you want to do this? All of these options are less-than-ideal, and require the server to be up and running correctly - they have many more possibilities for situations where you wouldn't be able to gain access to the box than, say, a good old KVM setup.
I don't know what your budget and space requirements are, but I bought a managed, 8-port KVM with scanning, server naming, hotkeys, the whole 10 yards for abour $50 on ebay. I bought a 21" CRT, keyboard, and mouse for $10 from a University's surplus store.