There used to be a HN meetup, but it stopped happening after a few months. A friend and I started Chicago.concat() which has been going strong for a few months and has a great group of people that show up every month. I have since moved to SF, but they're kicking ass back in Chicago. RSVP page: http://chicago-concat.gathers.us
We did a Chicago HN meetup around Christmas which was pretty well attended (filled about 1/4 of the Publican). I'd be game to do something like it again.
Lightbank and Sandbox have their own enclaves. The ITA (Illinois Technology Association) has offices, and some stuff. There is the lean startup circle. You can also out built in Chicago. Midventures also events. Entrepreneurs unpluggd seems to putting on more stuff, lately.
I'm one of the co-organizers of Entrepreneurs Unpluggd (http://www.entrepreneursunpluggd.com) and we plan on putting together events quarterly currently, if not more frequently.
Our inaugural event was Feb 8, next even is April 26, then late July after that. We really hope to create an evening for entrepreneurs to network, learn, and strengthen the ecosystem.