Thanks to simulation software, it's now possible to run historical software on the modern PC. Through the following links, you can get your own PDP-11, IBM mainframe, Altair 8800, and dozens of other fascinating machines. In many cases, the old operating systems are available. It's amazing really: you can now download for free what used to cost millions of dollars, and still have more free space left on your hard drive than all the disk storage in the world could have held back in the old days.
Here are some links to nifty emulation and preservation sites:
Here is a collection links to some popular emulators. If you know of others to add to this list, let us know! (We don't plan on listing game console emulators here, though... that's another world entirely!)