The only case I can remember, and I'm not even sure why I remember is it was used as a delimiter in the Runescape forums. Just checked and it still seems to be, eg http://services.runescape.com/m=forum/forums.ws?185,186,980,...

Sorry for the late response. I don't have an example, sorry.

Commas should not be used in URLs because they are so frequently used as delimiters in other scenarios. That's not to say that it can't work (so I'm sure there are some URLs with commas), but according to that RFC we shouldn't use them. Any URL with commas in it would be in violation of that spec. But hey, this is the web, we like to break specs all the time! ;-) It's my impression that this is more for sanity reasons than technical limitations.

The Onion has their article urls in the format:

http://www.theonion.com/articles/scientific-community-baffle...

/articles/slug,id/