Besides sharing files (for which Dropbox is great), I am increasingly symlinking folders to subfolders in my Dropbox to attain automatic synchronization between machines. It's just by far the most seamless and painless to set up solution I know of.

I recently became a paying user and I now store all my personal docs to my Dropbox. My main impediment to doing this was concern about security, but then I discovered and deployed Boxcryptor Classic (https://www.boxcryptor.com/en/boxcryptor-classic), which give me a great deal of peace of mind. It's a fully client-side, zero-knowledge file-system encryption app. I now keep all my documents in my Boxcryptor, which sits in my Dropbox.