I use Dropbox, DropSend and YouSendIt. IMO sites like divshare, rapidsshare, megaupload (well mega is gone) but anyway sites like these are very unprofessional and some artist will NOT download from them. For me it really depends on the artist. If I mixing for a major I NEVER use regular email because it can be easily hacked. In most cases I am mixing a song that has not been released yet so I can't afford to have it leaked. I only use YouSendIt or FTP direct to their server. With YouSendIt the file can only be opened by the email address that you assign to it.