Here are five of mine:
(1) Elvis Costello and The Brodsky Quartet: The Juliet Letters: I think just having "Elvis Costello" in a music collection automatically qualifies for this kind of list, and since it was in downloading this interesting "chamber quartet with vocals" compilation that I came up with this question in the first place, it seems a logical place to start. If you are one of the 25 or so people to actually buy this album, then cudos to you! Recorded in the early 1990s, this collaboration focuses on the premise that a Veronese professor has been answering letters written to the most famous Juliet in history (thank you, Mr. Shakespeare!), and then devises the "what would different letters to a current/former/hopefully lover" (or, in the case of the title character, a dead, imaginary woman) sound like? The result? Definitely different, strange, and not meant for the commercial airwaves.
(2) Bruce Springsteen, Nebraska: In high school, I didn't care much for Bruce. I didn't hate him, I just didn't especially care for him. And then, early in my college days, Nebraska came out. Just Bruce and an acoustic guitar. And a harmonica. And the most downright dreary, pessimistic lyrics I had ever heard, presenting sagas about an America gone to hell, yet still with a vision of what could be. And I was hooked. Since then, I've grown to love and appreciate Bruce Springsteen's powerful words, and have a decently large collection of his works. But Nebraska still ranks as an off-the-beaten-path recording; it may have been sandwiched between chart toppers The River and Born In The USA in the chronological order, but remains his most powerfully lyrical achievement to date ... in my honest opinion.
(3) Leonard Nimoy & William Shatner: Spaced Out! Most of you know I love works of parody. I own every Weird Al Yankovic album (even met him backstage after one of the three times I've seen him in concert), and Allan Sherman was a staple growing up. So it should be no surprise when I was walking the aisles at the now-bankrupt Tower Records store in Toronto in the early 1990s and happened upon this collection of ... um ... music ... that I was inspired to bring it back to the States. Shatner's crooning is horrid, and I mean this in the kindest terms possible. Nimoy's is reminiscent of Johnny Cash at times (including his cover of "I Walk The Line"), which is to say it's actually pretty decent. At times.
(4)Apocalyptica, Plays Metallica by Four Cellos: Do I really need to explain this one? Apocalyptica is a cello quartet. They recorded eight Metallica songs, including "Enter Sandman," "Master of Puppets," and "The Unforgiven."
(5) Lounge-A-Palooza: In the late 1990s, lounge music made a comeback. (I'm not actually sure if "comeback" is an accurate description, but it was at the same time that the Brian Setzer Orchestra and Cherry Popping Daddies hit the charts with the Big Band style, so it kinda-sorta fits.) The most interesting part of this short "rebirth" was this compilation, featuring (among others) Ben Folds Five (covering the Flaming Lips' "She Don't Use Jelly"), Pizzicato Five ("The Girl from Ipanema"), Glen Campbell & Michelle Shocked with Texas Tornadoes ("Wichita Lineman"), and ... Steve & Eydie ("Black Hole Sun"). Yes, when two icons of the 1950s cover Soundgarden, it's an interesting listen.
I'm sure I could add more, if I were considering more than five. I'm equally sure your list may vary But that's the fun of this venture, now ... isn't it?