Any of the Markdown apps - the above-mentioned Bear, Typora, Atom in Markdown mode,
Other apps - Symphytum, free edition of BBEdit in plain text or Markdown mode,
Any of the word processing apps like LibreOffice or WPS Office in Outline mode
What do I use most to keep track of notes? LyX, with each topic being a new and separate file/chapter.
Perhaps the easiest might be Typora, with each note topic a Heading. You can also insert a URL link, and if you right-click it takes you to the link. You would have to learn Markdown, but that takes about 5 minutes to become productive.
Symphytum is a very simple database system that you could set up to take notes with.
I just noticed that Dropbox has a Notebook mode, you could try that.
There are lots of things to try so you can find something that suits you. It really depends on your needs
- Do you just want to type down a few lines of notes with a heading?
- Do you want to include URL links and images?
- Do you want to include bibliographic references?
- Do you want to include web clippings?
Once you have answered that, you might have a better idea of what will suit your needs. It may be that one application is not suitable. I use Evernote for web clippings, Mendeley for full articles, LyX for written notes and Apple Notes for small snippets.
One problem with Markdown is that you can have endnotes or bibliography, but if you try to use both it gets a bit messy.
Don't forget Pandoc. This will let you convert many different document formats to many other different formats, for example Markdown -> PDF or -> ePub