IMSLP - A wonderful collection of public domain sheet music.  There are some very interesting first editions here.

Pianopedia - This site is very helpful when planning programs.  You can sort pieces by approximate length, year of composition, and even key. - Free concerts, plus €7.90 per month gets you unlimited access to a huge library of classical music documentaries and performances.

Naxos Music Library - One of the largest collections of streaming music online.  European subscribers also have access to historical performances by Horowitz, Gould, Gieseking, Cortot, Richter, Brendel and more.

Leon Fleisher Schubert Masterclass - Great teaching enhanced by technology. - This site contains public domain material which can be very interesting, from Schnabel Beethoven sonatas to books on piano playing by Josef Hofmann.

The Teaching Company - Greenberg's lectures on Beethoven's works are inspiring for music teachers looking to relate pieces to students. They go on sale every now and then.

Berlin Digital Concert Hall - A good option when you can't be there in person.