Noteflight. It's amazing.
It is completely cloud-based and is free, although (like I did) you can upgrade to a paid service for only $50/year or $8/month. It's learning curve is next to nothing. It is a flash component; in the process to converting to HTML5.
Just google it...check it out.
If you do decide to get an account, check out my songs! Username: challenger95
Thanks!