Notation software. That's the term I was looking for. I think that ConcertWare might have some functions that "fill in" your music, by automatically making chords and such, but I've never used it.
The method of inputting music is pretty clever if you're not a keyboardist - you use a graphical keyboard at the bottom of the screen to play notes, which then appear on your score. I recently bought a Creative ProdiKeys keyboard to try and speed this process up, but I'm such a poor piano player that I don't know if it helps or hinders me.
Here's a screenshot showing a work in progress:
http://www.msu.edu/~artmanja/cwscreen.jpgI might take a look at Personal Composer. The price is a little off-putting, but looking at the website, it does seem pretty feature-rich.