Just the other day, I had the Singing Mountain theme, from Chrono Trigger's OST (not actually used in the game), and it made me think, what bits of unused content do you like?

Another example I can think of is the Hidden Palace theme from Sonic the Hedgehog 2. The Hidden Palace level was cut from the game (incomplete versions of the level can be seen in the various prototypes floating around the internet), but the music written for it was coded into the game, and can be heard via the sound test. (The level, unfortunately, has been damaged to the point of being unplayable in the final version, even if you use a cheating device; the graphics are garbled, and collision data is missing so you fall to your death at the start.)

(Incidentally, that Chrono Trigger theme got used in the DS version, and I hear that the smartphone version of Sonic 2 includes a reconstruction of that Hidden Palace level, with a code that lets you play the incomplete version; also, many Sonic 2 romhacks include reconstructions of unfinished levels.)

So, what are *your* favorite bits of unused content in video games?

Edit: Why the low rating? Could somebody please explain what I am doing wrong here?

Not a video game, but your mention of music made me think of the theme music to the trailer of the film "Dark City". One of my favorite movies. The music in the trailer is the best part of the entire soundtrack...

...and it's not in the film anywhere.

Does missingno. count? I sometimes wonder what that pokemon was supposed to be.

Missingno.'s Japanese name apparently translates to something like "missing number".

Basically, each Pokemon has an index value that's used as an index into a table that has all the Pokemon in it. Thing is, there are 256 possible values for a byte, but only 151 Pokemon in the game. So, if the index doesn't correspond to a Pokemon, then either:
* You get a Missingno., which is a glitch Pokemon that actually has a name programmed in to act as a failsafe. (Note that the Pokemon is otherwise glitchy.)
* You get a Pokemon whose name is glitched (like the M-Block).

In any case, Missingno. is definitely a glitch Pokemon.

In later generations, you might see a Pokemon that looks like a question mark inside a circle, at least in 3rd Gen. That's the glitch Pokemon in that generation. (For example, in Emerald if your only Pokemon has 0 HP remaining thanks to a Pomeg berry, if you then enter a battle you will send out a glitch Pokemon that looks like that.)

I wouldn't count Missingno. as an unused Pokemon since it wasn't actually coded. On the other hand, I would count Mew, since that Pokemon can't be obtained in-game except via glitches. (Similarly, I would argue that the Minus World from Super Mario Bros. doesn't count, though it's at least interesting on the Famicom Disk System version.)

Edit: Speaking of Mario, Super Mario Bros. 3 actually does have some unused levels in the game data.

Voice lines. Always interesting to hear all that was recorded but never made it into the game or just don't play when they should for one reason or another. Just the other day, I've been going through all the voice lines for Sacred 2, and the amount of them that's completely unused or can pretty much never be heard during normal gameplay is simply staggering. So much time must have been wasted on recording them....

Every single NPC type has a full set of about 130 of them, including neutral and friendly ones. Ranging from combat grunts and yells to taunts and such.

I dunno, sometimes I just go to TCRF and hit random until I find something neat.

I love the industrial area soundtrack on the SNES port of Final Fight. It is still available in the sound test, but the stage was famously cut from the release.

I also like the unused neck snap animation in Tomb Raider: Angel of Darkness.

Sorry, I'd really like to, but... People usually get upset when I show them my "favorite bits of unused content" :'(

The Cut content from Knight of the Old Republic 2. One entire planet and tons of cut dialogue due to the publisher wanting the game out for the holiday season.

I suppose that having content cut because the game was in a rush is better than having the game be released in a buggy state, as happened with Final Fantasy 4 Advance.

By the way, the SNES Final Fantasy 2 (really a version of FF4) has a lot of dummied out items and abilities; turns out that they were used in the Japanese FF4 and are actually just leftovers. (Then again, FF4 DS has a couple unused augment abilities, one of which is called DoesNotExist.)

Then there's the mobile game Final Fantasy Dimensions, which apparently has a Blue Magic command.

Final Fantasy 2 (on the Famicom) has 4 unused tracks in its original OST. There's a shop theme (similar to FF1's), an airship theme (rather upbeat, like a typical FF airship theme; unfortunately this didn't fit the overall tone of the game), a boss theme (which some youtube comments say may have been too ambitious to the NES and could have used extra sound channels), and a dungeon theme (which was arranged and expanded for a certain FF6 art gallery dungeon).