NoteFlight is an online music notation tool (the first that I am aware of). Two options are available to users: a free account which allows up to ten scores, and a paid account (referred to as “Crescendo”) at $49/year or $7.99/month for unlimited use. These pricing options are significantly lower than the annual upgrade fees for Sibelius and Finale, but the web-based software of NoteFlight is less powerful than its professional counterparts. It does, however, offer increased convenience for those that want to work from multiple computers.
The largest deterrent for some individuals may be that this requires Adobe’s Flash Player. I believe this to be an unfortunate choice as it excludes many mobile computing options, such as iPads and many other tablets.