Transferring files over instant messenger is usually more miss than hit, and email limits you to 25MB at most—if you're both on Gmail. Streamfile facilitates streaming file transfers of up to 150MB, or 300MB with a free sign-up.
Hit-and-run file upload services like drop.io already offer file storage and sharing of up to 100MB, and more with paid accounts, so the main reason you'd pick a tool like Streamfile is its streaming powers. As soon as one party starts uploading a file, the other party can hit a link sent by email or other means and start downloading it under 256-bit encryption. Neither party requires anything more than a browser—not even a Flash plug-in, at that—and the links automatically die after 24 hours.
Streamfile offers larger file sizes (300MB), concurrent streams to multiple downloaders, and boosts the expiration to 48 hours for those who register with a free account, and even bigger files and wait periods for those paying $US4.95 per month. As the Download Squad blog points out, however, most anyone signing up for an account will want to head into their settings and un-check the options allowing Streamfile to send marketing messages or share your email with a third party.
Streamfile is a free service, sign-up not required.