Jamstudio is a cool web to create music tracks. Although you must pay for use it, ie, it costs $4/month, you can use a 10 minutes trial where you can explore a really big instruments online library and a usefull loop generator.
When you login with your email and your choosen password, you see something like this:
You have a pentagram with empty bars that you fill with the chord you want. You can create different loops, like:
G D Em C
G Em C D
Em Am Em Am
…
And then you can choose up to 8 different instruments. I think it’s a pretty good web.