The widely used herbicide paraquat is a viologen.  This application is the largest consumer of this class of compounds. The toxicity of the 2,2-, 4,4-, or 2,4'-bipyridylium-based viologens is related to their ability to form stable free radicals.  This redox activity allows these species to interfere with the electron transport chain in the plant.
Viologens have been commercialized as electrochromic systems because of their highly reversible and dramatic change of color upon reduction and oxidation.  In some applications, N-heptyl viologens are used.  Conducting solid supports such as titania and indium tin oxide have been used.