Ahavat Israel: Short background file
http://www.ahavat-israel.com/torat/purim.html

Aish Hatorah: Articles, Halachot & Traditions, music, videos, humour, kids' stuff
http://www.aish.com/holidays/purim/

Chabad: Play, read, listen, watch and do good deeds
www.virtualpurim.org
Facts, Saga, food, online events
http://www.purim.com/

Diaspora Yeshiva: Background, laws, commentary, cyber Purim play,
stories, games, recipes
http://www.shemayisrael.co.il/purim/index.htm

JewFAQ: Introductory background, additional reference files.
http://www.jewfaq.org/holiday9.htm

The Jewish Magazine: Article on meaning of Purim, and on customs
http://www.jewishmag.com/

My Jewish Learning: History, Halachot, perspectives, quiz
http://www.myjewishlearning.com/holidays/Purim.htm

Ohr Somayach: Background story, laws, deeper thoughts, humor, audio -
dozens of files 
http://www.ohr.org.il/special/purim/

Orthodox Union: Background, laws, messages, audio, rejoicing, recipes
http://www.ou.org/chagim/purim/

Torah Org: A wealth of themes discussed
http://www.torah.org/learning/yomtov/purim/

613: Jokes, advanced shiurim - dozens of audio files
http://www.613.org/purim.html

Virtual Bet Midrash: Shiurim from Har Etzion Yeshiva
http://freefind.com/servlet/freefind?id=5304605&pageid=r&query=Purim&mode=Match+ANY.

Virtual Jerusalem: Purim features and insights
http://www.virtualjerusalem.com/jewish_holidays/purim/index.htm

WUJS: Purim background, resources; Rosh Chodesh Adar [Women's issues]
http://www.wujs.org.il/index.php?option=com_content&task=view&id=223&Itemid=184

Hebrew

Kolech: Women in the Megillah and general resources