The Educator's Reference Desk (askeric.org)Excellent. Browse by subject, grade, keyword, phrase.
ScholasticSurprisingly robust; search by grade level, subject, keyword, themes. Featured and new lessons highlighted.
EDSITEmentSmithsonianEmphasis on "inquiry-based learning using primary sources and museum collections...print-friendly and provides you with all the materials you need—photographs, reproductions, handouts, activities, suggested strategies, standards information, and additional online resources" --SmithsonianThinkfinityKeyword search, subject, grade. Good descriptions.
EdHelper.comSubscription database ($19.95/yr) filled with lessons plans, activities, worksheets, printables.
MoreLinks to 47 databases of K-12 lessons. Links last checked August 2009.