NotePub is described as 'is the best online notepad, and it's free. Notes can be public or private and can include pictures, files, links, tags, and encrypted data. You decide who gets to see your notes, and who can edit your notes' and is an website in the Office & Productivity category. There are more than 100 alternatives to NotePub, not only websites but also apps for a variety of platforms, including Windows, Mac, Android and Linux. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Microsoft OneNote. Other great sites and apps similar to NotePub are Evernote (Freemium), Simplenote (Free, Open Source), CherryTree (Free, Open Source) and Zim (Free, Open Source).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.