For a writing utensil:
* Marqueur is the most common word for a marker.
For a marker as a point of reference:
* Marqueur can also be used here, meaning a "marker" or "signpost".
* Repère is another word for a "marker" in this sense, meaning a "landmark" or "point of reference".
For a marker in the sense of a "bookmark":
* Marque-page is the specific word for a bookmark.
So, the best translation of "marker" depends on what you mean by "marker"!