What is this effect called and what is the best free online tool to use to achieve this effect?
This effect is called a kaleidoscope effect (or Busby Berkeley effect in film). To add to user287001's comment, GIMP and Paint.net are great free options if you're comfortable using Photoshop. Another app you can check out is called PicsArt, which has a pretty specific tutorial on the kaleidescope effect here. Here's a few more examples if you search this term on Google: