I'm starting with Merlin, because I've already done a huge rec set and overview on crack_van, so you can use that to get started. Here are the links you need:
- Merlin: The Road to Camelot (Fandom Overview)
- crack_van's Merlin tag (for recs -- if you scroll back, I start with gen stories which introduce the characters, and then move on to Arthur/Merlin adventures and songvids)
- rec_room's Merlin tag (all my Merlin recs to date)
- The bbc_merlin_news community, which lists everything Merlin-related published each day.
There are, of course, many other fabulous links and communities out there, but you can find all of them from these starting points.