I had to downgrade from iOS 5 beta because this was broken. That's how much I love Dropbox.
Dropbox iPhone App
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better than the other apps I tried, awesome that you can send a temporary link to someone so they can download the file they need from you vs you downloading it, attaching it, finally emailing it. best of all, they don't have to jump through hoops to download the file (sign up for an acct f.ex.). but once they use it. they'll join too ;)
It's the best file sharing application available currently, and they continue to innovate, ship new versions quickly, and it allows for some pretty cool 3rd party additions that enhance scope and usability. a smart download! (NOTE: I've only used on iOS, other mobile OS/hardware may not produce similar results...)
5GB of free cloud storage. Automatically syncs across Windows, OS X, iphone, android and their website. Inviting your friends to create accounts can net you extra storage for free. Super handy for keeping all my documents synchronized across machines. Once it is setup you nearly never think about it again.