Is there simple note-taking tool available for Mac?
I recommend CodeBox. Simple interface with support for syntax highlighting, sync with Dropbox and etc.. Available in the Mac App Store
The PearNote app is one I can recommend with no caveats. It is simple, beautiful and powerful.
You don't have to use all of its features, but they are there if you ever wish to add audio or video.
Yes, it's called "Stickies" it's an App you already have on your Mac. Very simple but more powerful than most realize. Check it out.