Searching for a good way to exchange large files over the internet

It should be available for linux and windows (and if possible for mac).
It should allow transfers of files up to 5 GB.
It has to be P2P with the possibility to be sure files are not altered or intercepted.
The files should not be public but transfered privately.
Any suggestions. There is a lot of stuff out there, but it is not easy to choose.

have you tried the web site called yousendit i use this alot and its always fast

Hi Steve, thanks for the tip. Looked at it. This is good for a professional solution. And a bit costly one too. But I will hold it in mind for very important invoices and future solutions.
But it requires still a deposit of the file. I find this so curious. Is it so hard to connect to machines directly in a save way and do a transfer (even if it would take long)?
Seems that P2P is far from being developped.