Even though Google just released the Google Editions of the Galaxy S4 and the HTC One yesterday, Android developers have already broken down the firmware for both devices, as custom ROMs for the standard versions of both devices are beginning to appear. Along with custom ROMs, the new Android 4.3 Camera app from the Galaxy S4 has also been leaked.
Aside from some UI changes, the 4.3 Camera app seems to have the same feature set (including Photosphere) as the 4.2 Camera app. The leaked 4.3 Camera app is easy to install, as it can be installed as an APK. It also seems to be largely compatible with Galaxy Nexus, S2, S3, S4, Note, and Note II devices running on 4.1.x/4.2 TouchWiz and AOSP-based ROMs.
You can grab the leaked camera app from the XDA Developer Forums.