Justbeamit is a service I just learned about from Lifehacker (a great web site for resources and tips on everything, literally.)
With Justbeamit, you simply upload a file (drag and drop), get the private link and send the link to the person you want to share the file with.
No account sign up is required and it's very easy to use.
This is a great resource when you need to share a large file.
Some Other file sharing services:
Gett - http://educationaltechnologyguy.blogspot.com/2010/12/gett-gett-free-service-to-share-files.html
FileStork - http://educationaltechnologyguy.blogspot.com/2011/07/filestork-request-files-from-anyone.html
Fyles - http://educationaltechnologyguy.blogspot.com/2011/05/fyels-free-easy-simple-file-sharing.html
AVG LiveKive - http://educationaltechnologyguy.blogspot.com/2011/04/avg-launches-file-sync-backup-and-share.html
AirDropper - http://educationaltechnologyguy.blogspot.com/2010/12/airdropper-request-file-from-anyone.html
DROPitTOme - http://educationaltechnologyguy.blogspot.com/2010/11/dropittome-receive-files-to-your-inbox.html