Prompted by a question from my friend Maria, I think this is a good place to keep track of a list of places that have Cask Ale. Wikipedia, the source of all things half true (but totally on point here), defines/describes cask ale as "unfiltered and unpasteurised beer which is conditioned and served from a cask without additional nitrogen or carbon dioxide pressure. Cask ale may also be referred to as real ale, though that term is also applied to bottle-conditioned beer as well. Often this results in a creamier pour, with different head characteristics, and a subtle shift in the flavor of the beer. Hops become less sharp, and the malty sweetness of a beer gets to share more of the limelight. Often, you'll see the word "engine" or handpull associated with cask ale, referring to the device used for pumping ale from a cask.
What follows is an initial list from my head and some quick google searches. I'd like to include the suburbs as well, so bars/drinkers, please respond with more!
The List (to be reposted from time to time, as I discover more cask ale, or as bars remind me):
Owen & Engine - four beers on their "Engine"
Clark St. Ale House
Revolution Brewing Company
possibly Smallbar Logan
Goose Island Clybourn and Wrigleyville brewpubs
The Map Room
In Fine Spirits Lounge
Rock Bottom - Downtown/Near North
Bar on Buena
Mac's American Pub
Emmett's Ale House (Downers Grove)
The Onion Pub and Brewery (Lake Barrington)
Two Brothers Tap House (Warrenville)