The Internet Directory for Botany
An impressive series of links. A great place to start.
The Families of Flowering Plants, by L. Watson and M. J. Dallwitz
An ultra-detailed set of descriptions of all flowering plant families. Very complete.
The Angiosperm Phylogeny Group Web Site
The gory details of relationships among families and infrafamilial taxa. You must refer to this site.
The Plant List
Provides the accepted Latin name for most species, with links to all synonyms by which that species has been known. It also includes unresolved names for which the contributing data sources did not contain sufficient evidence to decide whether they were accepted orsynonyms.