Illinois Public Domain Land Tract Sales Database - for Stephenson County select this county and then enter the surname (at least first 2 letters) of the person whom you wish to search on  (a government database of over 550,000 19thC land sales in Illinois - info on obtaining original copies is here )
Genealogy Trails History and Genealogy Project - Pages for This County you will find user contributed records of births, deaths, marriages, military and newspaper extracts at this site
Stephenson County, Illinois Genealogy and History
Stephenson County Surnames
Stephenson County, IL Genweb Project a collection of online resources for genealogy researchers
The Tombstone Transcription Project - cemeteries for Stephenson Co. IL
US GenWeb Archives for Stephenson County cemeteries, newspapers, wills, military rosters etc recorded online

Additional Resources

Illinois Obituaries enter your Illinois ancestor's name to search obituaries in over 75 million newspaper pages
Illinois Birth Records
Illinois Marriage Records
Illinois Divorce Records
Illinois Death Records