Department of Natural Resources site provides information and photos on plants in MN (not as comprehensive as the Annotated Checklist - see Bell Museum below)
Access and download the 2010 "Annotated Checklist of MN Vascular Plants". Also has an identification section on grasses and an interactive identification key for MN woody plants.
Comprehensive site for plant information covering the entire United States. Search by Scientific or Common name. Best to use the genus and species name as many revisions have occurred to scientific nomenclature in recent years.