I guess, technically United would be the only one that would call Chicago Home, as that's where their corporate headquarters are, out by O'hare. Elk Grove actually.
But if you meant airlines that have pilot bases there:
United
American
American Eagle
Atlantic Coast
Air Wisconsin
I keep hearing rumors about a Skywest base there, but only rumors.
I don't think TSA has a base there do they?
Feel free to add any I forgot!