1,200 Weeds of the 48 States and Adjacent Canada on DVD (or even more species on flash drive). That’s a lot of weed species and covers most things one is likely to run into in North America.
Flora of North America
An important but technical reference intended to cover all the plants in North America. Seventeen of the planned 30 volumes have been completed. Print and electronic publication is available for most volumes. This is a hard reference to work with from scratch. Descriptions of families (and other taxa) are useful. In addition it lists many references for various groups of plants. Illustrations (except in the 2 volumes on grasses) are far from a complete coverage.
An online flora (and in print). Mostly useful in eastern Canada. Somewhat technical, but with excellent keys and comments.
An information retrieval system (DELTA) with an interactive key which allows free choice of characters, is “easy” to use, and can lead to correct identifications in spite of occasional errors. It can display illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications. The program and data are also available on DVD or thumb drive (see http://delta-intkey.com/www/datadvd.htm).
A multiple-entry key to the families of flowering plants as defined by the Angiosperm Phylogeny Group in 2009.
Key: Plant Families
This Android app is for both amateur plant enthusiasts and professional botanists. It is an interactive key to identify a plant to its respective family. You select characters and enter their values, such as flower color; the app will present you with possible identifications including family details and images.
This site has extensive descriptions of plant families (and orders), distribution maps and some images. It is mainly concerned with plant evolution and is not very helpful for identification, except to confirm family descriptions. It also has a long list of references which may be useful at times.