Fledgling musicians looking to ditch expensive composition software might want to check out Noteflight. It's a composition community that lets you put together musical scores right in your browser--that is, as long as you know what you're doing.
The good news is that the service's composition tools are wonderfully simple to use. Instead of requiring a MIDI keyboard to enter notes, you can simply plug them in with your keyboard and/or mouse. Each note can be modified with a simple drop down menu that follows you as you move around the page. It also contains an option to change pitch, note length, and the tempo. Advanced users can dig a little deeper with things like key and time signatures, and bar line styles.
What makes the service really neat is that you can see the work of others, and in some cases make changes. The sharing options let you toggle this on and off, and also lets you embed entire pieces elsewhere, like I've done below:
Noteflight's only real barrier to its collaborative editing is that you cannot send messages to other users directly (akin to e-mail). The only way to communicate with them is to leave comments on the score. There's also versioning support, meaning they can roll it back to a previous version if you've turned their masterpiece into an off-key version of "Chopsticks."
Below is a quick example of how to put together a Noteflight composition using nothing more than its tools.