Department for Jewish Zionist Education
A good introductory site with some in-depth material for educators.
http://www.jewishagency-ed.org/festivls/purim/index.html

Hagshama Department
Thoughts on Purim and its meaning
http://www.wzo.org.il/en/search/default.asp?search=purim&Submit=Search

Aish Hatorah
Numerous articles ranging from fun to in-depth
http://aish.com/holidays/purim/

Orthodox Union
Background information, customs and traditions and shiurim
http://www.ou.org/chagim/purim/default.htm

Project Genesis
Essays, divrei Torah and drashot on Purim
http://torah.org/learning/ (type PURIM in keyword search)

Ohr Samayach
Laws, customs, history, parodies
http://www.ohr.org.il/special/purim/

Ahavat Yisrael
Background, history, laws & customs
http://www.ahavat-israel.com/ahavat/torat/purim.asp

The Holiday Spot
Jokes, recipes, background food, cards
http://www.theholidayspot.com/purim/index1.htm