What is Belle?
Belle is cross-platform visual novel maker focused on being user friendly.
Despite of being in early stages of development, it's reasonably stable and provides many tools for creating complex visual novels and other 2-D games.
Belle is actually composed by two different applications:
Free and open source
Belle is free and open source software, both the editor and the engine are released under the GNU General Public License v3. This means you can modify this software and adapt it to your needs. It also means you can use it freely to make commercial or non-commercial games.
Belle's editor uses Qt4 as its base, so it works in all platforms supported by Qt4. Here are the main ones:
Belle's engine on the other hand runs on HTML(5), which means your games can run pratically anywhere!
Smartphones, tablets, laptops, desktops, all you need is a browser. 
Since version 0.3 alpha older browsers like Internet Explorer 7 and 8 are now supported aswell. 
Easy to use
Easy to distribute
Since the engine runs on HTML5, you don't need to worry about compiling and distributing libraries,
your games can simply be played online.
Of course there's also the possibility to distribute your games in a way so that people can play them offline.
1Modern browsers are recommended for a better user experience.
2Audio is still not supported due to the lack of HTML5 audio support. A fallback mode using Adobe Flash is being worked on.
3Distributing the game offline is perfectly doable, however the problem is some browsers, like Google Chrome, for security reasons, limit the browser's functionalities when running files locally. This means that any game, if run locally (opened directly from the index.html file), will not work in Chrome and some others. The easiest and best approach for now is to switch to a browser that has less restrictions, like Mozilla Firefox.