The Digital "DECSystem-10", one of the greatest mainframe computers in history. Most of the Internet software (TCP/IP, DNS, FTP, and so on) was written on these computers, which formed the original backbone of the ARPAnet (which later became the Internet). It's a 36-bit general-purpose computer with timesharing hardware, making it especially well-suited toward university student usage. Without the PDP-10 the Internet as we know it today may not have existed.
Oh Lord won't you buy me a PDP-10
My friends all hack VAXen, I must make amends
Worked hard all my lifetime, no help from my friends
So Lord won't you buy me a PDP-10.