I found this search tool for running events around Chicago..

For Chicago specifically, here are lists of events for the next two months.

Listings for April:

Running Network Calendar System link

Apr-05-08 Chicago Lakefront 50K George Cheung Memorial Race Chicago IL
Apr-06-08 Run for the Cheetah 5K Chicago IL
Apr-13-08 Race to Wrigley 5K Run Chicago IL
Apr-26-08 Path to Progress Chicago IL
Apr-27-08 Universal Sole Lakefront 10 Chicago IL
Apr-27-08 Ravenswood Run Chicago IL

Listings for May:

Running Network Calendar System link

May-01-08 Race for a Safe State: A 5K Run, Walk and Roll Chicago IL
May-03-08 Run To Remember Chicago IL
May-04-08 Chicago Distance Classic Half Marathon Training Program Chicago IL
May-04-08 C4 Miles: Making Strides for Mental Health Chicago IL
May-05-08 Polish Constitution Day Hash Chicago IL
May-06-08 Interval Track Workouts for Women Chicago IL
May-11-08 Y-ME's Race to Empower Chicago IL
May-18-08 Heartland Spring Chase Chicago IL
May-18-08 Alternate Route 8K Race Chicago IL
May-22-08 JP Morgan Chase Corporate Challenge Chicago IL
May-24-08 Fleet Feet Sports Soldier Field 10 Mile presented by adidas Chicago IL
May-25-08 Chicago Hash Memorial Day Hash Chicago IL
May-26-08 Ridge Run Chicago IL