We love ButtonBass because it's so easy to create an awesome-sounding song. Just pick the type of music genre you want, select some cubes, and build your beat. You can then record your song using on online recorder like Vocaroo or RecordMP3Online. Here is a student project that used ButtonBass to create a rap.
The Tony-B Machine is a cool online synthesizer that lets you play a song with your keyboard, while you add beats, effects, and even a few vocals using the buttons.
Incredibox is probably the most fun to use, but the songs you create may not be as useful for your students. Just drag a hat onto a singer and they will change the sound they make.
You can download free music to use in projects at BenSound or IncomTeach (or there are many other sites... just do a Google search for "royalty free music."
DJ Turntable is a cool site where you can upload an MP3 file and then change the speed or add special effects. You might also check out the Drum Machine while you're on the site.
And finally... you definitely want to be an Ugly Dancer!