When I started teaching Chinese, there were very few Chinese language learning resources available online. I searched frequently, hoping to find whatever might be of value to me. One day, almost by chance, I found a corpus of Chinese. A Chinese university put up a search engine that allowed the user to enter a word and returned as many sentences with that word in as it had. This was a tremendous boon for me as it provided authentic examples of words in use. Over the years, some of the best resources have just kind of fallen into my lap, though my lap had to be in the right place.
Today, both the quality and quantity of resources available online, many for free, is effectively limitless, at least in the sense that you cannot possibly see and use them all. Many sites help teachers find these resources, but even beyond these there is lots to discover.
In addition to sites specifically created for teachers, many teachers create their own websites and offer their materials to others. With the relatively recent advent of Creative Commons licensing, where content creators intentionally license their materials for others to use, the amount of content and the means to use it have grown substantially. There are also sites such as Khan Academy, which offers free video lessons. We can categorize the types of sites that provide resources specifically applicable to teaching as follows, with an example of each. Note that this categorization is my own and by no means standard.
Individual teachers' websites (Mrs. Moore's wiki)
Material Sharing websites (Sharemylesson)
School websites (Kinard Science)
Open Educational Resource websites (MIT Open Courseware)
Open Course materials websites (OER Commons)
Resources for Teachers websites (Edudemic)
Professional Organization websites (ISTE)
Instructional Materials websites (Khan Academy)
Tool websites (Geometer's Sketchpad)