Personally for quick, simple and easy prototyping MarvelApp is great, also the free version has unlimited project number, so that's awesome too. For more complex and cool looking prototypes I suggest http://www.pixate.com it also has a pretty good free plan and lot's of features that are only about creating a more complex prototype.
Personally for quick, simple and easy prototyping MarvelApp is great, also the free version has unlimited project number, so that's awesome too. For more complex and cool looking prototypes I suggest http://www.pixate.com it also has a pretty good free plan and lot's of features that are only about creating a more complex prototype.