If what you are looking for is basic point-and-click music notation software, NotePad is reasonably good. It's free, and it gets the job done for basic stuff.
You can't do anything like key/time changes, and you can only do basic rests. Grace notes and trills are out, as well.
In addition, it's a bit of a drag to run, at least on my old iMac, as it tends to be slow and less responsive than one would hope.
So, in other words, great, if you want to write out "Twinkle, twinkle, little star", but not good for much that's longer or more complex. But it's free, and it's point-and-click. You get what you pay for.
If you need more advanced capabilities for typesetting, I highly recommend LilyPond (just do a macupdate search). It uses text-based input to create beautiful, complex scores, and, is therefore (provided you are willing to learn the syntax) an excellent alternative for those who would like free, advanced music notation software.