Where is Xcode's build folder?

~/Library/Developer/Xcode/DerivedData is now the default.
You can set the prefs in Xcode to allow projects to specify their build directories.

It should by located in: ~/Library/Developer/Xcode/DerivedData.

If you changed the defaults, you can see where the build directory is by going to File->Workspace Settings then look at Build Location

You can configure the output directory using the CONFIGURATION_BUILD_DIR environment variable.

Source: http://developer.apple.com/library/mac/#documentation/DeveloperTools/Reference/XcodeBuildSettingRef/0-Introduction/introduction.html#//apple_ref/doc/uid/TP40003931-CH1-SW1