I think that there is a free buffet on the second floor of El cantinero on University place. They don't have it every day of the week though, mostly on Thursdays nights and fridays also. The frozen margaritas are 4$ and they are incredible. They are strong and good! The food downstairs is good and not too expensive.
Salute on 38th and Madison has free apps on bar during happy hour....like focaccia pizza squares, dried meats, olives, etc.
Pampano, 49th bet 2nd & 3rd (thanks for the address correction designerboy) had some great apps during happy hour that are served on a three tier plate rack. I don't remember what was served, but it was good.
I don't know if they still do, but you can give them a call. No harm done. The bar at Keen's is very nice. During happy hour they serve complimentary BBQ spare rib tips. They usually set a heaping bowl down at several places at the bar and anywhere a circle of people might be gathered (at the long counter opposite the bar for instance).
I remember Keen's most because not many places put out ribs during happy hour. The other places are not so memorable. Could be what I was drinking that affected that.