The best source to make sure you know about openings and events is OnTheMake.org.

New to me, but also fairly comprehensive is The Gallery Crawl and so much more.... And, there is also Paul Klein's Art Letter, a consistent staple to Chicago.

National websites with listings for current exhibitions in Chicago are Artforum.com's ArtGuide, Re-title.com and ArtSlant.com. Locally, there are fairly comprehensive listings in these news sources: New City, TimeOut Chicago, Flavorpill, Chicago Reader. There is also the city-organized multi-genre chicagoartistsresource.org's Calendar.