Info on pricing, side effects, and warnings/interactions:
http://www.walgreens.com/library/finddrug/druginfosearch.jhtml

Info on psychiatric meds specifically:
http://www.crazymeds.org/

Clinical and prescribing info on lots of meds (suggested by TodaysWonderful -- thanks!):
http://www.nlm.nih.gov/medlineplus/druginformation.html