MyHabit is a part of Amazon and they have a great promotion going on.

Refer friends to MyHabit and you can get free credit to spend!

You get $5 for getting 1 friend to sign up
$25 for 5 for getting 1 friend to sign up
$50 for 10 for getting 1 friend to sign up

Make sure to spend it, the free credit expires 6 months after you earn it.

Click here to sign up