You might want to look at a good FTP client like Cute FTP
I believe they have a version that's someone "host" files on their system and grant access to a user group for upload and download of files.
Then you just need a good plan for collaboration, so nobody undoes something a partner did.
One method I have used, Zip up the local site folder. Upload to web server or email it to the next team member. In addition, use a meaningful file name each step of the way: XYZsite-Apr-07.zip.
Another solution might be to install Tiki Wiki (group collaboration software) on your server.
This is an update for anyone following this discussion. Neither of the two suggestions offered so far seem to be the solution I'm looking for. I have, however, been looking at two other possibilities. One is to use Windows Office Live. You get 5 gigs of storage and the ability to share files easily online. There are also other benefits such as the ability to add comments and some project management capabilities. Another option I've been researching is a European based company caled Live Drive. They offer online storage and another option called briefcase. Any file that you put in the briefcase is available from any computer with an Internet connection.
I haven't comitted to either strategy yet. So if you have experience with either of the solutions mentioned thus far or have a better one, please share.