OS X :: Move Large Files On The Web?

I do a lot of work that involves advertising, photography, video editing, DVD authoring, photoshop and illustrator files. The illustrator vector files are fairly compressed but the photoshop raster files can be extremely large. Sometimes, I have someone wishing to send me a large portfolio folder, or I find I may want to send a big file to someone else. As an example, I recently had an artist send me a hi-res portfolio of his book covers to be made into a promotional DVD for comic-con. One of the CD's he sent me had the wrong pics on it, so I had to wait for the correct pics to be sent, which delayed the project several days. I can compress a single file for emailing, but can not drop the resolution as a way to minimize folder size if the file is going to be printed. I work predominantly on a mac, but many of my customers use windows based operating systems.

What is the best method for sending and receiving large digital files from a variety of customers? If I get an FTP site, can I keep one client from accessing someone else's folders? I noticed that Skype allows for file transfer... is VOIP practical for sending large files? This month's macworld made mention of a bit torrent program for such a chore. What about using a bit torrent program such as Transmission for creating a bit torrent folder for moving large files to a single customer? Are paid services available? Is a paid service the best choice? I am looking for a method that I can both send and receive these large files. It should be easy enough for a less than tech savvy customer to send me files after I give them a minimal explanation of how to do it. Preferably, it would involve no cost to a customer, and no need for them to install software.