I still remember the todays when I was still in college where you having to work with other students to create a web page using XAMPP stack. But before we work on it we had to come up with some wireframe work, I did it in Mockingbird. It seems like a good tool back then, but things has changed dramatically, sites like Pixate, Marvel,Invision, and others (I am sure there's a lot more) started to change how we did prototyping and/or wireframe. The only problem is pixate is mobile only (Well, only android and ios, no windows phone) but pixate is very powerful especially with those gestures and animation where you can show users what happen when you swiped left or right. Marvel is the new guy on the block, that offers a lot of features that you would found in Invision, but it has room to grow. A good alternative if for some reason anyone dislike invision. Invision is the most powerful one, it offers live sharing, comments, group chat etc.

Invision live sharing demo

Group chat on the bottom left for discussion, comments on the bottom right for anyone to add comment on any page. You can navigate different demos using the arrow keys. The highlighted boxes are intractable to make it work like the real live application on mobile.

Invision seems to offer the most amount of features and make prototype a lot easier. But the other two are pretty good alternative and especially pixate, after playing around I felt like pixate offers more powerful mobile support than invision, but invision has more to offer (has web, wireframe etc).