Not just for smartphones... but I use Dropbox.

The #1 thing I use it for on my phone is putting pictures on my PC's. I just put the pictures in the dropbox and they're instantly available on any PC that i installed dropbox on. Saves the time of hooking up the phone via USB or emailing them to myself. Works with all file types.

If you use my referral link you get an extra 250 MB of storage space for free: http://db.tt/JivAJ2x (i do to, so i'd appreciate it if you used my link )