Dropbox (3 GB free) and iDrive Sync (5 GB free) are good starter systems to get your files back and forth.
 
Another option that's finally available is Gobbler for PC (it's been available on the Mac for a while now). It also offers 5 GB for free initially, but it's supposed to be geared more for swapping large media files.
 
Steve

For your file transfer, you could also get a hosted web site for pretty cheap, and use FTP to upload/download files to each other through that - FileZilla is free for instance, as FTP software.

I don't know if that would be cost effective, but is something you could look into for comparison.

Bob Bone

Has anyone mentioned any cap that me be applied to your ISP account? I know that it is more common nowadays to have an unlimited download/upload amount, but it might be worth checking the small print, just in case there is a daft clause in there.

Having said that, I have a 40gb cap on my account and my household has doubled that in the last two months and I've not heard from my ISP yet... So, perhaps they don't care...

I agree with Razorwit about allowing each participant to use his/her own preferred platform.  It may also be worth joining an online collaboration site (e.g. Indaba as mentioned by Michael) and using its facilities to communicate and exchange stems.