With Gmail, or Yahoo Mail, you can send files up to 10 MB, which is pretty large (a 10-minute mp3 file, for example).
But sometimes, you need to send someone a file that is larger. My favorite service is YouSendIt. I send files to Boston and get them back fine with this service. Normally, a geek like me would use FTP, but that requires owning a web site and using specialized software. With YouSendIt, it is all done on the web, and is very similar to sending an email.