Here's a question that I want to investigate: is King Features Syndicate the only American comic strip syndicate around today that still publishes company-owned strips?
I'm going ignore something like the Spider-Man strip or other strips based on licensed properties for sake of this argument.
I know Peanuts and Garfield were both originally owned by United Feature Syndicate, though they have since gone back to their creators' companies. I think Bill Watterson now owns Calvin & Hobbes, but I don't know all the details off the top of my head.
EDIT: it seems that the E.W. Scripps Company still owns some of the UFS properties, even though they outsourced the distribution to Universal Uclick:http://www.comicsbeat.com/universal-uclick-to-syndicate-uniteds-comic-strips/