One of the perks of being a Nexus device includes Google provided, factory restoration images. Today, Google has made Android Jelly Bean 4.1.1 (JRO03L) available for the global variant of the Nexus S (soju) as well as Android Jelly Bean 4.1.1 (JRO03R) available for the Nexus S 4G (sojus). These will do you good if you happen to flash a bad ROM or do run into issues tweaking and customizing your Nexus device.
Sadly, the Nexus image download site is still missing Jelly Bean images for the Korean version of the Nexus S as well as both LTE Galaxy Nexus images. Hopefully, those images appear soon and updates start to roll out for those still yearning for Android Jelly Bean action on their Nexus phones.