Hype from Tumult is fantastic and actually builds everything with html and css. Super powerful and fairly intuitive. http://tumult.com/hype/

Works really well for advanced interaction animation prototyping as well as just setting up general flows. etc. And since it's building everything with standard web technologies you're not limited to testing on specific platforms. The app makes it simple to export to a Dropbox folder for easy sharing of links that can be viewed on pretty much any web browser as well as exporting as GIFs and other options. That's one of the problems with apps like Principle, no support for any mobile platforms except iOS.

Unfortunately, even with LFS support, Git doesn't work well for design files. The problem is that every time you make a change to a file and commit it it keeps the history stored locally on your machine. Because these are binary files it basically has to just duplicate the entire file every time which leads to a ridiculously large local repository very quickly. LFS lets you store those files somewhere else rather than on your local machine (the repository will just have some metadata instead unless you specifically request the files), and that's great for devs that don't actually need to work on those files, but for designers who need all the files available to work on, it just isn't feasible.

If anyone knows of a good way around this I'd greatly appreciate it as we would love to start using Github more for all of our design files but so far we haven't been able to make it work. Using Dropbox until that happens or something else better comes along.

If you have a very small team Pixelapse looks pretty good. Unfortunately they don't differentiate between designers and observers like LayerVault does so it gets expensive really quick if you want to be able to get feedback from more people. They've been acquired by Dropbox but say they'll have a migration path when the new stuff is done on Dropbox.

Invisionapp.com is reasonably priced and has all the features you want I think. The thing I really dislike about them is they have a very rigid file/folder structure that is limiting. Also each project has to have a specific type (desktop/web, iPhone landscape, iPhone portrait, iPad, Android... ) so you aren't supposed to mix even iPhone and iPad mockups in the same project. Subfolders aren't allowed anyway so it wouldn't work well even if you didn't have to choose from those options. Very frustrating but outside of that it seems to work well and has some nice features.

Thanks so much for all the awesome comments everyone! I'm the lead designer for 1Password. Really appreciate seeing so many other designers using and liking the app I work on every day :)

Marvin (marvinapp.com) is by far the best I've tried. I like it better than Readmill personally. Only thing it doesn't have is the social stuff.

I use and love it. It has made it much easier for me to keep our developers in sync with my latest changes and get quick feedback without any extra work on my part.

I use the Dropbox sync method so that I don't lose out on Dropbox's easy folder/file sharing features. There were initially some problems with it (as expected with a major and complex new feature) but LayerVault's support was excellent at getting things fixed quickly for me and I haven't had any issues at all lately.

No degree and doing great. I definitely got lucky at a few points in my career though.