First and foremost the album that I have had on repeat for about three days now: ToToM's mashup, With Boots. Stunning. Free download. The songs "We're in this Blurred Line Together," "Another Spot in the Sun, Part 2" (with Pink Floyd) and "Only Fame" (with David Bowie) are amazing. The album does a great job of further elaborating on the poppy and the gothic influences evident in Reznor's work. Also, hearing Kool and the Gang repeat "Get down on it" while Reznor sings the chorus to "Only" is something I will treasure.
Jonny Cash. "Hurt."
"At the Heart of it All" by Aphex Twin. Off of 26 Mixes for Cash. At the time it was a great look at the electronic influences Reznor so prominently displayed on "Every Day is Exactly The Same." The song is still unbelievable. All the quirkiness of Aphex Twin with the raw beauty of Nine Inch Nails. I love it.
And finally, Reznor's remix of David Bowie's "I'm Afraid of American's." I like it and I like it a lot. Reznor has often said he has been majorly influenced by Bowie, and I like it every time they get together, although the concert going fans don't seem to - in 1995 the Outside Tour, which co-headlined both NIN and Bowie, was canceled early because nobody bought tickets.