Midjourney Artist Influence Reference Guide: Aaron H's excellent guide uses artist prompts with various media and subjects. (Aaron and I collaborated on this one -- v5 update coming soon!!)
MidLibrary: Fantastic and thorough (though slightly chaotic) site with thousands of genres, artists, techniques, and styles.
WillWulfken's Midjourney Styles and Keywords Reference: Another huge reference guide, using spheres as reference points. Appears to be in the process of updating for v5.
Prompter by Shane McGeehan: This is a Google Sheets document that you'll need to copy and save to your own Google Drive to use
MidJourney Prompt Inspiration by T. L. Ford (Slate#4353): Prompt builder that includes visual examples for numerous styles, artists, etc. Also includes random generators.
TokenizedHQ: A good intro to creating prompts.
Troubleshooting Midjourney Text Prompts by Clarinet: Great guide to the best phrasing for prompts. Updated for v5!
Clarinet's Puppet Method: Learn to use the same characters in different images. (Also updated for v5.)
Prompt Chat: Fantastic thread with lots of helpful folks who can help you troubleshoot your prompts.
Prompt FAQs: Full of helpful hints and links.
img2prompt: Upload a photo and see what prompts you could use to create a similar image.
CLIP-Interrogator: This does the same thing, with slightly different results.
Kris Kashtanova shares fun prompt ideas, offers daily challenges, and shares articles on AI on Twitter.
The Midjourney subreddit is chock-full of posters' hilarious and creative images.
Midgard's Theory of Layer-Separated Prompting is for use in ChatGPT and has really upped my prompting game!