- CAAHEP
- Cain Brothers
- Calamos Asset Management, Inc.
- CAN Financial
- Cano Container Corporation
- Capsonic Automotive Inc.
- Carl Sandburg College
- Carle BroMenn Medical Center
- Carle Foundation Hospital
- Carle Health Methodist Hospital
- Carus Corporation
- Caterpillar
- CDWG
- Center for Exhibition Industry Research
- Chicago Bears
- Chicago Marriott Northwest, Hoffman Estates
- Chicago Medical Society
- Chicago Police Department
- Chicago Public Schools
- Chicago Southland CVB
- Chicago State University, College of Health Science
- Chinese American Service League, Inc. - Adult Day Care
- Christopher Rural Health Planning Corporation
- City of Chicago
- CMC Electronics
- Coastal Hotel Group
- Coe-Truman Technologies, Inc.
- College of American Pathologists
- College of DuPage
- Collinsville Drury Inn
- Community First Medical Center
- Community Health Emergency Services
- Community Health Initiative
- Community Memorial Hospital of Staunton
- Computing Technology Industry Association
- Cook County
- Cook-Witter Inc
- Courtesy Ford of Danville
- Crowne Plaza Springfield - Convention CTR
- Crusader Community Health
- Cub Scout Pack 194
- Current Works, Inc.
- Dearing Headquarters Company, A Headco Company
- Decatur Memorial Hospital
- Delta Dental of Illinois
- Dental Assisting National Board
- Department of Energy, Chicago Operations Office
- DePaul University
- Devnet Inc.
- Discover Financial Services