I'll put a more detailed list come October since I want to do a full article, but there are alternative writing challenges and websites. Normally I'd recommend 4thewords since I use that, but they've switched from a pay-as-you-go site to subscription. They do writing challenges and events, though.

Here are some links with alternatives:

https://www.reddit.com/r/nanowrimo/comments/1bqref6/looking_for_alternatives_for_the_nano_website/

https://medium.com/illumination/nanowrimo-alternatives-bcab90d8c512