Beepbox is "an online tool for sketching and sharing instrumental music.". I'm not going to go into detail on how it works here, as you can just scroll down on the website to see a tutorial; but i am going to tell you that it is a LOT more capable than it first appears to be: You have free reign over it with keyboard shortcuts, and you can make unlimited patterns and make your song as long as you want. There's even less limitations than that if you use one of its many mods (did i mention it's open source?), like Jummbox, which has even more powerful keyboard shortcuts and even modulator tracks!