Please check also this geospatial data search tool: http://maps.lib.uic.edu/

Introduction
Obtaining GIS data of Illinois could be hard, depending on the data. Low-resolution (small scale) research data are relatively easy to obtain, for example, natural resources data from ILGS. On the other hands, large-scale data, such as public registry data (for example, parcels & property data) and administrative data (for example, zoning) are generally hard to access in IL. Followings are some of datasets available from our collection and also online resources for geospatial information. For more information on geospatial data, please contact us at lib-maps@uic.edu.

Library Collection
Selected data collection at the UIC Library - Documents - Maps Section.

NIPC regional spatial datasets - what is it?
(Published years: vary)
UIC-affiliated member access only. NIPC (Northesastern Illinois Planning Commission) creates large-scale geospatial data for various planning purposes. Most datasets available from NIPC, such as regional base maps, 1990 & 2001 landuse inventories, are available for the UIC affiliated members. Please contact us at lib-maps@uic.edu for more information about this dataset.

City of Chicago GIS files - what is it?
(Published years: starting from 2005)
The UIC Library collects and archive historical GIS datasets available from the City of Chicago GIS website. Please contact us at lib-maps@uic.edu for more information about this dataset.

1978 Chicago Neighborhoods - download this file
(Published in 2005 - see below)
This is a 1978 Chicago Neighborhood boundary file based on a map published by the City of Chicago. The source of the neighborhood information is a survey conducted in 1978 by the City. The UIC Library scanned the map and then created boundaries based on the scanned/georeferenced map.

Web Resources

Illinois

Dept. of Natural Resources - Illinois Geospatial Data Clearinghouse
http://www.isgs.uiuc.edu/nsdihome/ISGSindex.html
Developed and maintained by the Illinois Geological Survey (ILGS), this GIS data clearinghouse provides one of the most comprehensive geospatial datasets that cover the entire state of Illinois, including scanned maps and digital aerial photographs. Most are ready to be used for GIS analysis, though, some are in the E00 (ArcInfo interchange) format. Importing E00 files in ArcGIS - Import E00 files in ArcView 3.x (use "import71 utility" - search help file for more information).

(Great Lakes Region) Upper Midwest Environmental Science Center
http://www.umesc.usgs.gov/
The Upper Midwest Environmental Sciences Center (UMESC) links its research closely with other U.S. Geological Survey (USGS) science centers to broadly address ecological and population concerns throughout the Nation's heartland. The majority of our ecological research is conducted in support of Department of the Interior issues and lands in the Upper Midwest.

Illinois State Geological Survey
http://www.isgs.uiuc.edu
Check also their data collection website, http://www.isgs.uiuc.edu/isgshome/datab-coll.htm. 

ILGISA: Illinois Geographic Information Systems Association
http://www.ilgisa.org/
ILGISA is an association for professional GIS users and analysts in IL. 

Illinois Geographic Information Council
http://www.illinois.gov/ilgic/
ILGIC was created in 1995 to promote coordination of geographic information development and the associated technologies. ILGIC is hosted by the IL Dept of Natural Resources. Check also Inside Illinois, which is a collection of web links to the GIS/map related local governments' websites.

Metro Chicago

NIPC: Northeastern Illinois Planning Commission
http://www.nipc.org/
NIPC produces and provides statistical/spatial data for the metropolitan Chicago six counties (Cook, DuPage, Kane, Lake, McHenry, and Will). Data are available from NIPC by request.

Natural Connections: Green Infrastructure in Wisconsin, Illinois and Indiana(A.K.A. Green Mapping Project)
http://www.greenmapping.org/
Download a variety of geospatial data files for the metropolitan Chicago area from this very comprehensive data clearinghouse. This is a collaborative effort by non-profit organizations and local government to share the natural resources related spatial data for anybody who is interested in environmental analysis. Data are available both in the ESRI shapefiles and MapInfo TAB formats.

Northeastern Illinois Counties

Lake County: GIS and Mapping Division
http://www.co.lake.il.us/gis/default.asp
The Geographic Information Systems (GIS) and Mapping Division of the Department of Information and Technology provides a variety of geographic products and services to county departments, other government agencies and the public.

DuPage County: GIS Division of Information Technology
http://www.co.dupage.il.us/GIS/index.cfm
Starting in 1980 a GIS. program was initiated to make the county government entirely self-sufficient in meeting all mapping and GIS. data needs.

Kane County: GIS-Technologies
http://www.co.kane.il.us/gistech.asp
The GIS-Technologies Department was established in 1998, it is responsible for the development, distribution, maintenance, integration and training of GIS software applications and datasets.

Cook County: Assessor's Office
http://www.cookcountyassessor.com/filings/gis.asp
It seems to me that Cook County does not have so-called "GIS department" per se, however, the (tax) Assessor's Office has a long history of using GIS to manage geospatial and property information. Other departments that use GIS at Cook County are: Dept of Office Technology, Cleark, Highway, ETSB/Sheriff (E911 system), Health, Forest Preserve District, Zoning Board of Appeals, and Elections. I believe this is a good start to locate GIS data created by the county.

City of Chicago

City of Chicago: Geographic Information Systems, Business & Information Services
http://egov.cityofchicago.org/gis
Under the Business & Information Services Department, the Geographic Information System workgroup creates and manages geospatial databases. Many of GIS data are available for downloading (check under "GIS Data"). Also see several interactive mapping sites to obtain more spatial information.

University of Chicago: Chicago GIS data
http://www.lib.uchicago.edu/e/su/maps/chigis.html
"The material was created by the staff of the Map Collection at the University of Chicago. The material may be freely copied, but you should most definitely read the caveats in the notes for each section. You should attribute authorship to the Map Collection, University of Chicago Library, and to the personal authors mentioned in the texts." - from the website.

University of Illinois at Chicago: Download Subsidized Housing Information
http://www.lib.uchicago.edu/e/su/maps/chigis.html
The Illinois Assisted housing Action and Research Project (IHARP) data available from here in Excel file can be easily geocoded to be used for GIS analysis. Please check our "how to geocode data" site.