The Perseus Project has the entire Lewis & Short online in a searchable form. This used to be a precious resource that most Latin students could ill afford, but now anyone with an internet connection can access it. They also have many texts online.
The Latin Library also features a number of texts, including Medieval Latin and some neo-Latin.
Whitaker’s Words is a program that can parse a given Latin morpheme and tell you what word it is, which is often difficult especially for beginners because some words take on a different form in the perfect system. Here is an online version. There are several front-ends for Whitaker’s Words. My favorite is Legible Latin.
Laura Gibbs does a blog about Latin, with daily features such as a word-of-the-day, various mottos-of-the-day, etc. She also collates Aesop’s Fables from various sources and makes them available online, including in forms specially marked for reading practice in either the classical or the ecclesiastical style.
Many books which were precious and rare posessions once upon a time are now scanned in and available online, including Latin dictionaries, grammars, readers, etc. In addition to the Internet Archive, as Panurge pointed out, Google Books has many rare books available to download, as well as pre-views of more recent works still in copyright. Project Gutenberg has a handful of Latin resources, and my favorite would have to be The Handbook of Latin synonyms, which expounds on the denotative and connotative force of many similar-seeming Latin words.
There is the Vicipaedia: Wikipedia in Latin.
That’s just the stuff that might be of direct interest to someone not looking to get involved in the Living Latin scene, where there are blogs, bulletin boards, social networks, contemporary poets, ect. I myself have been translating some out-of-copyright materials into Latin, since I don’t believe you can know a language’s full vigor from just its most high-minded materials. You need loads and loads of trash to read.
