For me, there's only 2 valid websites (not apps). Unlucid AI and MotionMuse.
Unlucid AI, you just follow this link
and the instructions of my tutorial:
» My AI Creations (and a small tutorial)
Then motionmuse... it's even easier, but it's more difficult to do free examples. You can collect with patience 5 points each day. The same, follow the link, register with gmail.