/Users/<YourUser>/Library/Caches/com.apple.dt.Xcode. I wouldn't usually mess with this sort of internal caches, but in this case it looks like XCode won't remove old files after installing new ones: I had three .dmg files with different versions of Cli-Tools. I removed all .dmg files except latest version, and it doesn't look like I broke anything, so it's probably a safe trick. In my case, I claimed back some 250 MB -- not huge, but coupled with cleaning all caches for my secondary browsers (done from the proper UI), I got back about 1.5 GB today.
04 March 2013
I'm always looking for ways to reclaim space on my crammed hard disk. Thanks to the lovely JDiskReport, I've found a little directory where XCode stores the files it downloads when you choose to install components from the Preferences screen: