At the time of publication of this article the Chicago Tribune provides an archive of past issues from 1849 through 1991 that are text searchable. You may also type in a date. The images are displayed for free. This Beta capability has been available for awhile. Thanks to my friend Larry Parker who made me aware of this.
This may not always be the case. When the archive is officially launched as an online searchable database, it is likely that the display of the images from search results will be behind a pay wall.
But for now, if you have ancestors in the Chicago area, I would highly recommend that you take advantage of this Free (currently) resource.
The Chicago Tribune once had a slogan of "The World's Greatest Newspaper". This was on Page 1 from 1911 to 1976. Did you know that radio station WGN, purchased by the Tribune in 1924, stood for World's Greatest Newspaper? I didn't, and now I have one more clever fact that I will remember always.
To search the Tribune archives, go to http://archives.chicagotribune.com/
You might want to read the FAQ section to understand what the beta is all about and their plans for the future.