https://stephenhoerner.com/knote/
This is an online notepad with KaTeX and Markdown support. One day, while taking math notes on Google Drive, I realized how useful it would be to have the power of LaTeX math in a simple, lightweight, cloud-based application.
It hooks into Dropbox's Datastore API, making it convenient for users access, outsourcing the storage of data, and allowing for some nifty features—for example, real-time updates across devices.
Unfortunately, KaTeX is currently rather limited. Pull request acceptance appears to happen slowly, so it may be a while before the implementation is satisfactory. (I have a pull request pending at the time of writing).