Ken
Thompson’s Comprehensive Endgame Database
Perhaps a bit advanced for beginners, this site offers a complete analysis
of piece endings. The interface is easy to use, though, and I think anyone
will learn something from just playing through the lines.
Shakmaty
Bereolos by GM Peter Bereolos
Excellent blog of the Tennessee master, full of analysis of his recent games
and instructional commentary on endings (including corrections of various published
endgame analysis).