Dropbox -- sign up for it by clicking this link and you will get a free additional 500MB! (And I will get a free additional 1GB for referring you.)
Dropbox was first to a lot of this stuff, and is continuing to expand its features very impressively. Save stuff on your desktop just by putting it in the Dropbox folder. It's sync'd to wherever you add the Dropbox client/app.
I also use Google Drive, same sorta thing except I dunno if there's an iPhone app.
And now Google Keep - for to-do lists and such, very handy. Write a shopping list on your desktop, have it available on your tablet and phone, and vice-versas.