The one I use the most is called "Melody Assistant" by MYRIAD in France. You can download a free trial version with most of the bells and whistles disabled, but it will give you the idea, and it's only $15 US to register it, with free upgrades. Some folks say that it will do nearly everything that the big professional programs costing several hundred $ will do; I don't know as I hae never used the big - time ones, but it will do about everything I need to do and then some.
I'm no good at HTML code to create a blicky, but here's the URL for MYRIAD:

http://www.myriad-online.com/

here is a "freebie" which I have downloaded but not fooled around with much - but you can compose with it and I think it will play your score back for you as well:
"Finale Note Pad"

http://www.hitsquad.com/smm/programs/FinaleNotePad/

It is an interesting little utility and seems to have potential.

Enjoy your inspiration and I hope that it's a big hit!

Uncle Jaque up heah in Maine