The main point IMO of the markdown format was that it is supposed to be easy to read and not require a specialised editor. Nonetheless, people inevitably end up wanting one, because we want to integrate documents syncing, or mathematics preview, or image linking, or note taking etc.
There are many, both integrated into normal text editors and more specialized editors; see e.g. this review for an overview of some interesting ones. In particular some of the note taking systems use markdown as the backend and double as markdown editors.
RStudio has a neatly integrated markdown editor especially for RMarkdown documents, which I also use.
Typora is another one that I’ve seen that seems popular. Available, like the previous two, for windows, macOS and Linux. Does look pretty and highly polished.