A downloadable tool for Windows and Mac OS X
Opus prototype is a tool to generate infinite landscapes by combining noise artifacts. You can use the editor to create a world and use the library to import it to your own application. Opus prototype offers you a way to easily describe landscapes (like seen in Minecraft) with values that are based on pseudo random noise algorithms.
With Opus you can explore one way to create procedural content and it may inspire you for your own games.
Note: this editor and the library are prototypes. The sources of the editor and the library are open source and available at https://github.com/aphex-/opusproto
The planets of the game Alien Ark where made using opus.
The editor is based on libGDX
Take a look at the documentation that comes with the editor.