NanoStudio

Software for music composing is usually expensive. Nevertheless, NanoStudio is a free program with lots of resources for professional composers and serious amateurs alike.
The program's interface is deceptively simple.

It only features an upper panel divided into two spaces, and a lower panel showing a keyboard. The upper panel is divided into two graphic channels that add effects to the notes played on the keyboard. You can use the latter to compose melodies and rhythms, and record them to play and edit them. The program also includes several backing tracks that you can use as a basis for your compositions.

There are other options too. By clicking on the upper buttons, you can select an equalizer, as well as other effect consoles and instruments.

Although the interface is very intuitive and almost anybody can start playing music immediately, the main disadvantage is that the program is not for casual users. You need to have some music knowledge, as well as some experience in using this type of programs. If it's not the case, then you will need to study the manual in deep and do a lot of practice to take avantage of all the program's features.