I just realised I haven't recommended Dropbox here yet. I usually have a few complaints about an app, no matter how good, but Dropbox is pretty much magical. I don't know what it could do better. For me it was one of those things I didn't know I needed.
Fancy introduction video if you don't feel like reading.
If you do:Basically it's a magical folder on your computer (and phone if you want). Put something in it, and it's automatically synchronized to your online Dropbox account (in the background; you won't have to think about it). If you have another computer with Dropbox, when you turn it on you'll find that your files are in the Dropbox there, too. Same with your phone, obviously.
It automatically provides an online backup for your documents. It also keeps a history, so if you deleted or overwrote a file that you needed, just go to your dropbox, locate the file and download an earlier copy.
It lets you share folders with people. You can invite other Dropbox users to whichever folders you want. It's a great way to keep a "Photos" folder together with your family, sharing a folder with colleagues or classmates for your current project or just sharing files with a friend for fun. If two people are modifying the same file simultaneously, no worries - Dropbox notices and saves one person's work as a copy.
There's also a web interface where you can access your files from any computer with internet - it makes USB sticks and emailing files to yourself seem kinda redundant.
If you feel like trying it out, please use this link as it gives me some extra space for my account