You have plenty of options for notes apps out there. If you're looking for something that you can host yourself, however, Laverna is an open-source notes app that works right in your browser.
Laverna bills itself as an open source alternative to Evernote, but it's not nearly as complex as that. Instead, it's a pretty simple notes web app that supports Markdown, task creation and encryption. You can install Laverna on your own server or use it at the link below. Laverna then saves all your notes inside your browser or in Dropbox for syncing. It's a pretty simple setup, but it works well for what it is. It's also nice that you can install the software on your own server so you don't need to worry about losing access at any point if it goes out of business.