Stanford University
Stanford Encyclopedia of Philosophy: Stoic Ethics
An extensive discussion of the ethics of Stoicism. Very detailed, includes lots of definitions of Greek words necessary to the philosophy.
Other
Zeno of Citium: Founder of Stoicism
A biography of Zeno, founder of Stoicism. Also discusses the theories of Stoicism. Included a nice set of quotes from Zeno, passed on through other ancient authors.
Other
International Stoic Society: Zenon of Kitium
A biography of Zeno and a discussion of his Stoic beliefs.
Internet Encyclopedia of Philosophy
Internet Encyclopedia of Philosophy: Stoicism
This site from the Internet Encyclopedia of Philosophy provides a good introduction to the main ideas of Stoicism. Contextualizes the claims of this school among Aristotelian, Heraclitean, and Cynic traditions.