Seneca - Wikisource has all the best online translations for Seneca. I recommend starting with On the Shortness of Life and moving on to the Letters to Lucilius.
Epictetus - The Enchiridion (or handbook) translated by Elizabeth Carter. This is a summary of the discourses, sort of a greatest hits collection. Caution should be used since without context some of these can be easily misunderstood.
The Discourses (alternate) translated by Elizabeth Carter. This is the best online version I've found, Elizabeth Carter was the first to translate Epictetus into English and used modern English in the 18th Century. Naturally more recent translations might be preferable but they are not in the public domain.
Marcus Aurelius - Meditations translated by George Long. A version I like better is Hutcheson and Moor's translation available in a PDF. But neither translation compares to Gregory Hays' (Modern Library Edition) or Robin Hard's (Wordsworth Edition) modern translations.
www.reddit.com/r/stoicism - Reddit is a link aggregator and allows votes and comments on links or text posts. There is a relatively active group there exploring issues and information about Stoicism.
Yahoo group - Email group that has existed since 1998 with archives available.
New Stoa - Includes the Registry of Stoics and the College of Stoic Philosophers, an online course with one on one mentors.