From the official website (user translation):
The 1w6 System is a light and universal role playing game released under a free license.
Its playing style is ideal for groups which desire fast and clear results for their actions and clearly-defined characters.
The basic rules fit on a single sheet on paper, can be extended with modules and have been tested about once a week since 2004. You can (and are allowed to) use them for your world directly, change them and redistribute them.
And they use only one die, the ±D6.
If that sounds interesting, check out if the rules fit your playing style.