The time has come friends. Factory images are now available for several Nexus devices. The current factory image (JRO03[C-E] depending on the device) is available for most Nexus S variants, though the Korean and Sprint versions are conspicuously absent. Similarly, the Verizon-branded Galaxy Nexus is still off the list, but all other Galaxy Nexus versions are accounted for. And, of course, the carrier-less Nexus 7's factory image is available.
For those who prefer bullet points, here are the devices with factory images available as well as the build number for each:
Nexus S (soju): Android 4.1.1 (JRO03E)
Nexus S (sojua): Android 4.1.1 (JRO03E)
Galaxy Nexus (yakju): Android 4.1.1 (JRO03C)
Galaxy Nexus (takju): Android 4.1.1 (JRO03C)
Nexus 7 (nakasi): Android 4.1.1 (JRO03D)
If you need to get back to the way things were when you first got your device, you can download the images from Google's download site right here.