Let's say you want to provide an example url in documentation somewhere. The correct thing to do is use example.com, since that is the defined location for such things and anything else you come up with is likely to be actually registered somewhere and serving up who knows what to your readers.
My questions is, what do you do if you need to provide 2 contrasting examples? The best I've seen is site1.example.com and site2.example.com, but that doesn't always cut it. Any other ideas?
Use example.net and example.org.
Per RFC 2606, they are all reserved (Wikipedia summary).