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.
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.