Third Watch to Without a Trace

ER's Dr. Lewis went to New York to look for here sister and was aided by the characters of Thrid Watch.

A reporter on Chicago Hope talked to Dr. Austin about having been accepted into the space program and mentions an ER doctor from County was also accepted. In a separate episode of ER Dr. Greene was accepted in the space program.

Chicago Hope's Jeffrey Geiger accepted an organ of a victim on Homicide

Law and Order and Homicide crossed over on four episodes

Multiple episodes feature Hudson University, the Alma Mater of multiple L&O characters