the standard google cardboard app was fun to use.. but the real utility for me is being able to load and view my own 3d models.. using sketchfab.com
all you have to do is go to one of your models(or any model) and add /embed?cardboard=1 at the end of its url
when you put the nexus in, remember to shift it a little to the side so that the center of the screen(where the two views join) is aligned to the center of the cardboard viewer.
note: by going into fullscreen mode it overrides the launching of the google cardboard app(triggered by the nfc tag)
This particular Google Cardboard kit is by Knoxlabs.. http://www.shoplocket.com/products/VYkE3-google-c... i ordered it right after the Google Cardboard popped up on youtube .. now you can find it all over amazon and ebay for a bit cheaper. You can also buy just the lenses and magnet, and build your own housing.
UPDATE Dec 2016:
Since i made this instructable, sketchfab has made a few improvements, one being that you can use the cardboard2(non magnetic button) button to click and teleport to different parts of the scene.. so you can go closer, or walk around a model.
Additional gamepads and controllers have limited support. more on that here - https://sketchfab.com/virtual-reality you can see teleport in acton here - https://sketchfab.com/virtual-reality
My Suzanne Robocop model can be viewed at https://sketchfab.com/models/0bff507c203044b8aa4b...
and downloaded for free at http://www.blendswap.com/blends/view/70048