Copy is my favorite.
In all honesty, it is basically a complete ripoff of Dropbox with 2 advantages.
1. More space. You get 15GB free to start and 5GB for each referral with an unlimited number. As you can see below, I have quite a bit of storage myself
Never paid a dime. Just sent out a bunch of referrals all over the web. MacRumors doesn't allow it, so if you want a referral so you can start with 20GB free, send me a PM
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This added space is great for syncing photos and videos, which Copy does automatically through what they call "PhotoCopy", which is like Dropbox's "Camera Upload" feature.
2. "Fair Storage" is probably their most unique feature compared to Dropbox.
Say you are working on a group project with 5 people. You invite a team mate who then invites another team mate and so on. You all have 20GB. Well for your group project if you have a shared folder, you actually have 100GB to work with between you all. So not only does Copy give you more space than anyone, but they also let you use it more efficiently than anyone.
And of course, like Dropbox, you have no file size limits or anything like that. Works on Android, iOS, Windows Phone, Mac, and Windows (not sure if they have a Linux app). It really does work like Dropbox.
And the desktop app lets you undelete files which can be pretty handy.
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I definitely recommend Copy over anything else. If you want an invite, let me know.