Every now and then, car manufacturers will come up with an engine that, for whatever reason is a bit more special. These engines often find their way into deserving chassis, resulting in great cars. Sometimes they even end up in multiple chassis, spreading their goodness far and wide. The last generation Celica for instance, wasn't a bad car, but it's engine went on to power the diminutive Elise, a car better able to exploit it's sky high redline and revvy nature.
Can you imagine though, if instead of the Celica and Elise, Toyota had simply stopped after shoving the 2ZZge into the Pontiac Vibe and Toyota Matrix? It would be a phenomenal waste of a pretty good engine. But unfortunately this happens all the time. Here's a list of some of my favorite engines that never really made it in the automotive world. Engines that I wish had come in more cars.
side note: I live in the U.S. so these engines are all from USDM cars. Apologies to the rest of the world and I'd also love to hear your opinion and thoughts about these engines or cars.
1996-1999 3.4L V8 from the Taurus SHO - 235 hp

http://www.youtube.com/watch?v=40ay52N4SYE
Why I like it: Yamaha makes damn good engines and this is probably one of their rarest. The V8 was a scant 3.4L making it pretty tiny and smaller than the V6 i've got in my car. Usually V8s of that size are found in Ferraris.
Why it's wasted: The 1996-1999 Taurus SHO was both FWD, and A/T only. It was also heavy and lets face it, not the best chassis in the world. It was also one of the ugliest cars to come from Ford in a very very long time.
Where I'd tell them to stick it: This is tricky because Ford really didn't have a chassis at the time that could exploit this engine. The Mustang had it's own V8 at the time that was much bigger and more powerful (on paper.) Ford did have an association with Mazda however and Mazda did just stop importing the FD due to people hating rotaries. It would never happen, but it would be kind of cool. In a perfect world, this engine would power 2nd generation MR2s.
1992-1996 1.8L V6 engine from the MX-3 - 130 hp

Why I like it: Continuing my theme of "tiny for their cylinder count" the K8 engine in the MX-3 is one of the smallest V6 engines to ever find its way into a production car. It had a 7K redline, but the fuel cutoff wasn't actually until 7800 RPM. It was similar in size and power to many of the higher performance 4-clyinders of the day, however it had them all beat in refinement.

Why it's wasted: The MX-3 was actually a pretty decent little car. It did look like a bean and it was FWD, but truthfully it wasn't half bad. The problem is that the K8 engine is just so damn cool it deserves to be in more than one car sold for only a few years.
Why it's wasted: The MX-3 was actually a pretty decent little car. It did look like a bean and it was FWD, but truthfully it wasn't half bad. The problem is that the K8 engine is just so damn cool it deserves to be in more than one car sold for only a few years.

Where I'd tell them to stick it: I think the answer here is pretty obvious; this should have been an optional engine on the Miata. The MX-5 was making about 115 horses at the time and I think Mazda's engineers could've probably pulled another 10-15 horsepower out of our little V6 here. The result would be a slightly more refined gran turismo style Miata. But of course it would still have a tiny engine that pulls smoothly to the upper reaches of the tach. In short, it would still be a Miata.

1992-1996 3.3L Flat 6 from the Subaru SVX - 231 hp

Why I like it: Flat sixes are always kind of cool, but the EG33 engine in the SVX was no slouch either. 231 hp from an NA engine was pretty dang good for the early 90's and in fact, Subaru wouldn't build a more powerful NA engine until 2008. Plus, its design lends it's self to a good center of gravity, which would  be helpful in something with more serious sporting pretensions.
Why it's wasted: The SVX, like the MX-3 wasn't a particularly bad car, it just wasn't hard core enough to do justice to its engine. It's well documented that the SVX came equipped only with an automatic, which is it's self a great tragedy. Furthermore Subaru refused to equip the SVX with the much more sporting center differential from the Japanese version of the car, meaning that the AWD SVX was essentially FWD most of the time. In fact, the BRZ is the first car since the SVX to be offered with 2WD; as a desperate attempt to cut SVX base prices in 1994 led to a short lived FWD model.
Where i'd tell them to stick it: I would have liked to see a 2.5RS coupe with this engine. Obviously it would have to be a 3.3RS coupe, but a sportier impreza with just a bit more refinement than the WRX could've been really cool. The WRX could then be brought over in four-door-only form as a wanna be rally car, while the 3.3RS would be the grand tourer.

Alright, i'm missing a ton of great engines, but those are my top three as far as underutilized. What do you guys think? Chime in below.