What are the major online databases of anime, manga, light novels, visual novels, and other related media? What are the differences between them?
To qualify as an answer, the source should list a large fraction of series in some particular medium, not just one or a few related series. The primary purpose of sites here should be indexing information about series. While it's acceptable to link to sites that do have some information related to e.g. fansubs, the primary purpose of all links in answers here should be indexing, not providing download links. Also, databases which are out-of-date should be removed.
Non-English sources are welcome so long as they qualify, but please specify what language in parentheses.
Note: This is intended to be the canonical reference for questions like this to avoid lots of new questions coming in with slightly different specifications. Please check the sites here to see if any fit what you want before asking a new question. The answer is CW, so feel free to add to it.