Playground

Pros: Has plenty of filters to use as well as different style of art to choose from, on the plus side the free version gives 500 image day limit.

Cons: Lack of respect for creators, you can't express your own creativity without being told off by the morons that run this site by showing image NSFW, any attempt to create this image will result in blah, blah not to mention images with too many fingers, arms, legs etc.

Overall: Easy to use, you can have 500 images on free plan or 1000 for their pro or 2500 for their turbo plan, but I am not going to keep giving them money if they don't allow users to express their creativity.

Pros: It's free to use.
Can use your creations commercially.
Has many features.
Easy enough to use.
Quick way to download.
Uses (part of) the prompt as filename.
Can use an image as a base from which to work.
Shows settings after rendering.
Keeps last used settings.

Cons: Takes a while to learn all the features.
Main screen is a bit difficult to 'control' - images don't snap into any order.
Step limit of 50 is too low.

Overall: A very useful tool once you have learnt all it's secrets, especially if you have an image that you use as a base; then you can even use it without a prompt.