I'm more or less assuming all of us are familiar with the Apple Lisa, Apple's and Steve Jobs' first attempt at turning Xerox PARC's work into a marketable product. It was a flop, but many of its ideas carried over onto the Macintosh, and in fact, Macintosh development took place on the Lisa.
Due to the fact few Lisas were sold, it's hard to get your hands on a working model, meaning most of us will never get the opportunity to actually use one. Luckily, there's a very advanced Lisa emulator available, written by Ray Arachelian. It's open source (GPL) and available for Windows, Linux, and Mac OS X. Setting it up is remarkably easy (there are non-hqx Lisa operating system files too), but do note that technically, you need to own a Lisa yourself in order to use the ROMs. But, of course you do.
The Lisa user interface takes a bit of time to get used to, as its terminology is a bit alien, and some things feel quite a bit arbitrary from our modern, harmonised perspective. For instance, it took me a while to figure out how to open a new file in the Lisa office applications; it turns out that you need to double-click a special kind of icon ('stationary'), which the Lisa calls 'tear off stationary', which creates what to me looks like a copy of said stationary icon. This icon is the actual new file; double-click it to open it.
I'm having quite some fun poking around the Lisa, and the emulator has been holding up perfectly - no crashes, and it's remarkably easy to use. I would definitely advise giving this a go if you have some spare time.