In light of the Federation of Genealogical Societies Conference currently underway in Springfield, IL, I thought I would highlight a few Illinois sources. First, like Missouri, Illinois has a great state website that has numerous links to the state's archives and digitized records. You will find the search engine for the databases here. The archives include digitized military records for Illinois residents including the Civil War, Spanish American War, Black Hawk War and War of 1812. Many of the databases are index only so a fee may be required for access to actual records.
In addition, the Illinois State Genealogy Society Blog shares updates and information that will help your search. Happy hunting!