Excellent dichotomous keys which are helpful in most of the Upper Midwest. The graminoid keys are especially helpful, although the taxonomy is not always up to date.  

Often very out of date, but also the best online keys available for many groups and regions.

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High quality photos and detailed accounts for most vascular plants found in Minnesota, including helpful 'similar species' sections.

Best source for county-level distributions, but also has many other tools. Can be out of date and inherits errors from individual herbaria.

Flora of the Southeastern US. The definitive source for a large & very biodiverse corner of the continent, including up to date taxonomy.

Digitized collections from dozens of Midwestern herbaria. 

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