The Midjourney Prompt Builder is a good option, with thousands of options for art styles, colors and objects. It's geared for people who want to create something new and unusual, with image-to-text prompts and Repeat and Permutation options to create a group of jobs from a single prompt. It's available on the web, Android and iOS, so it's good for both creative pros and casual users.
Another option with a lot of power is ArtRoom AI. This AI tool lets you create, edit and refine images with a variety of options like AI art generation, full-service painting and ControlNets for fine control over imagery. It also comes with a gallery of different styles and mediums, and several pricing tiers, including a free option, so you can pick what's right for you.
If you want something more flexible, starryai is a good option. You can generate up to 25 images per day for free, with a library of more than 1000 styles. The app also offers reimagining existing images, fine-tuning, and a powerful API for developers who want to build it into their own projects. It's good for casual users and pros who want to experiment with AI art.