To use this service, you go to the transmission page, and enter the email address to receive notification of the file available for transfer. The recipient then gets an email, containing a URL on YouSendIt's servers. I just transferred a small PPT file and it's available at http://s11.yousendit.com/d.aspx?id=BF2CA0E2E83AC30C57A9E9D731C378E8. It will get deleted within 7 days (i.e. by 10 July!).
The idea is cool, and I can certainly see a use for it. But there are a couple of interesting questions. First, what is the business model for these guys? I mean, what is going to be their return on letting me mail someone 1gb of data at a time for free? While I like free, there really is no such thing as free these days - someone's got to pay for the servers and the bandwidth needed. Does the adware on the site pay for this? The second question is over security of data. While I don't mind sharing a picture or two - and having it out on the internet - but what other use might be made of this information or data? How do I know that it will only go to the people I actually send it to? The site has a nice security policy, and offers transfers via SSL. This latter feature is one that makes me vaguely nervous as it could be a potential attack vector, bypassing perimeter and network defenses.
What ever the answer, this service does seem to work, and as long as it's free, it's a neat way of sending large files to people across the internet.