Incredibox
An easy-to-use web-based program using loops to create your own version of a song. You can record your song and see if it hits their Top50! You must have Adobe Flash to use the site (which means it will not work on Android, iPhone or iPad).
MusicTheory.net
One of the best music theory websites out there! The site provides comprehensive from basic note reading to advanced chordal analysis. Use their apps for ipad and smart phone "Theory Lessons" and "Tenuto."