Judging from volume, Bitcoin-24 is the new reference.
Thanks Grinder.
I found
this page listing exchanges on the Wiki, but it doesn't have any indication whether or not each exchange is operating at the moment. I don't know how to contact the folks who might be willing to let me write some code so that each bitcoin exchange can have a ping or info URL and the wiki can use it in order to show people the current state of each exchange. I'm interested in doing that, so if anyone reading this knows whom I should contact, let me know.