Illinois Public Domain Land Tract Sales Database - for Mercer 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
Mercer County Almshouse Register Index (1859-1948)
Mercer County, IL Genweb Project a collection of online resources for genealogy researchers
The Tombstone Transcription Project - cemeteries for Mercer Co. IL
US GenWeb Archives for Mercer County cemeteries, newspapers, wills, military rosters etc recorded online

Additional Resources

Ilinois Newspaper Search enter your ancestor's name to instantly search the world's largest newspaper archive; over 12 million newspaper pages
Illinois Birth Records
Illinois Marriage Records
Illinois Divorce Records
Illinois Death Records