This website has free games that allow you to practice note identification and fingerings for your instrument. If you get all the answers in your game right, you get rewarded with a snippet of music and have the option to go to the next level. If you get one or more answers wrong, it tells you what your mistake was and recommends you try again.
If you explore this website, you can listen to live performances from the Millennium Stage at the John F. Kennedy Center for the Performing Arts, which is located in Washington DC. Every day, they broadcast a live concert for free (and you can attend them for free, too!). Their website has archives of all their live performances, so it is a fabulous way to listen to/watch some amazing musicians of many genres. You can also browse their Digital Stage, which has shorter video clips that you can search by art form (including ballet, opera, international, jazz, and theater).
This website offers a wonderful exploration of jazz, which is what many people say is one of the few truly American art forms. You can listen to famous players and hear jazz charts, learn about jazz history, and watch interviews with jazz musicians who talk about their experiences with music.
This website is maintained by the Dallas Symphony Orchestra. You can do a variety of things on the website, like learn about instruments (and listen to what they sound like), learn about composers (and listen to some of their most famous works), and get instructions to build your own instrument.
This website offers free music notation software. You have to create an account, but that is free, too. Once you have an account you can notate (that means write) your own music using their user-friendly notation tools. It automatically saves what you are working on, and you can even listen to your compositions as you go.
This website has free music theory lessons, as well as many exercises to practice various aspects of music. One of my favorites is the rhythmic dictation exercise (under the music theory and ear training exercise tab)!