Lots of players on here know and love this site for tactics training:
These 2 sites are very good for basic endgame training, esp. the 1st one:
The 2nd one only has the "basic checkmates"; the 1st one has those and more advanced positions, such as Lucena, Philidor, queen vs. pawn on the 7th, and others, and also has great explanatory videos for about 20 different positions. Also, the 1st one seems to play the best moves based on tablebases whereas the 2nd one seems to use an engine that often doesn't make ideal endgame moves--but it's still good training.
(I'm writing this as a setup for a question I have in Thread 112515