Brahms' Rhapsody in G minor is wonderful, the other one in the set is good too.

One of my favourite pieces of all time is Beethoven's Pathetique Sonata (1st movement here).

For more contemporary stuff, I can't recommend Ligeti's solo piano music enough. Etude 13 is in large part responsible for me getting into contemporary music, and thus, becoming a composer.

EDIT: Also, John Field was a big influence on Chopin; he's credited with inventing the nocturne form I think. Not as virtuosic though, if that's what you're into.