Here's another one to add to the list: 

https://zoo.replicate.dev/  New to me, this one uses up to seven different models at once, Stable Diffusion XL (which wasn't working for me at at the time I tested it), Stable Diffusion 2.1, Stable Diffusion 1.5, DALL-E, Kandinsky 2, DeepFloyd IF and Material Diffusion. It's interesting to see the results you can get from these different models while using one prompt.