My iPhone 4 says it has 15 GB of "Other" in the storage bar in iTunes. What is it and how do I get rid of it?

you can try

http://support.apple.com/kb/HT1414

This has officially got to be the worst response I've ever seen - it's not even an answer

"Other" could mean text messages, settings, etc. It could also be an issue, sometimes iTunes doesn't detect your device properly and just show one big bar across the bottom of how much space is taken up on your iDevice and just label it as "Other," if iTunes is persistent about this, then I would try using iTunes on another computer, or try to uninstall/redownload iTunes to see if it resolves the issue. (Does iTunes just show "Other," or does it show other bars such as "music," "videos," "apps," etc.?) If this problem is causing any errors/issues with iTunes, or the phone, then try restoring the phone (Make sure to backup!), and then restore from backup, and then see if iTunes still does the same.

In my case, this must be me. My iPhone has a bug - it's not any of the other things that are considered "other". It's just an issue. I've researched for over an hour, reduced every settings imagineable (Other doesn't go down - in fact, it goes up, almost like, because I freed up space, it wants to fill in the remaining space). I called Apple - after one hour of being inadvertently hung up on twice (they took my number to call me back in case disconnected and never did - amazing) and then talking to another 3 reps, they just "guessed" which I find offensive to my time - just say "I don't know but if you are willing to take a chance, here's a thought..." rather than not be able to say "I don't know". Warning: Resets DO NOT solve this problem unless you do a clean wipe out and start from scratch (w/o restoring your stuff). Two hours later, I've resolved to wipe out my phone.

All of your content should already be in iTunes on the computer.

At most you will lose message history and call history.

It's not the end of the world.

At least not until December 12th according to the Mayan's.

So chill out... restore the device and move on with life.

Uh.. I believe the main thing is app data, which is very valuable to some people. And also the pictures (but those can be copied and pasted), but app data cannot be extracted from a backup easily and without special programs that cost on windows and are free for Mac.