There are so many options you can choose from. I think the MIT license is the best option (and my personal favorite license), take a look: http://en.wikipedia.org/wiki/MIT_license

There are also the zlib, BSD and MPL license which offer the same degree of freedom. Other alternative would be to release your app as public domain or as any of the Creative Commons license, for example: http://creativecommons.org/licenses/by/3.0/