There is a free service being offered by Texting Forward and the Bank of America Chicago Marathon. This will allow people to track up to three runners in real-time during the race, receiving notices as their runner(s) reach each timing mat along the course (representing their 'split times').
Split Times for the Marathon this year will be at the following distances: 10K, Half Marathon, and 30K. A notice will also be generated when the runner crosses the Finish. Line. Note that these times are considered 'unofficial' even though they are provided by the official timing company for the race. You can have these notices sent via text message to your mobile phone, your Facebook page, or your Twitter account.
It is my understanding that this service will be published on the Chicago Marathon web site this Friday. But the direct web site for this tracking service is https://tadpole.textingforward.com/portal/tf_marathon_login_social?pvRaceId=1481
For those who do not wish to sign up for this service, you can go to the Chicago Marathon's web site on Race Day and search real-time results right from there. This should work for anyone via a computer, or who have a web-enabled smartphone or tablet. For those at the race, the Marathon will also have kiosk stations available for runner tracking as well as tracking locations in eight selected McDonalds restaurants.
Information about these and other spectator-related information/services can be found on the Chicago Marathon web site's Spectator Page.