Dropbox: Dropbox allows you to share files and documents easily between computers and mobile devices by using the Dropbox apps. Simply place your files in the Dropbox folder, and they will be synced to all of your Dropbox-enabled devices. You can also choose to share a Dropbox folder with someone, and that person will then be able to access the files within that folder. Free to use.
Evernote: Evernote allows you to "remember everything" by syncing to-do lists, photos, definitions, vocabulary terms, and anything else between your computer and your mobile devices. Free to use.