If you have always been interested in trying a system with an old unix, but did not want to research how to install any emulator, someone has created a PDP11 emulator in javascript: http://pdp11.aiju.de/
Everything runs in your browser.
If you want a guided and versatile experience on old computers, do not miss going into the impressive page of Telehack.
This page is not a totally real simulation, it is more like a game, but it offers an experience similar to what it would be like to fiddle (hack) with machines and networks from around 1980.
Source: http://thenewsh.blogspot.com/2011/05/retro-cmd-line-fun.html