I’ve used Piano Library a lot recently to find new Classical piano pieces to play.
What I particularly like about this site is that it ranks works by difficulty (and even better by composer AND difficulty), so that I can quickly assess whether a work is my within reach technically : I pick a composer I like, find pieces I can already play and see what other pieces are ranked at the same level, or slightly higher. The icing on the cake is that it also has audio samples from the first few bars of each work, so that I can decide immediately whether I like one enough to spend time learning it.
Is there something comparable for jazz standards ?