Though not strictly for gaming, Dropbox is a great free service that I use regularly to share files with my gaming group. Dropbox gives you 2 gigabytes of free storage space and an application to keep directories on your computer synced. Since Dropbox has a nice system to restrict access to specific directories, all you need do is create a “game” directory and drop campaign updates or character sheets in and it will automatically be downloaded to everyone allowed access to that folder. Other users cannot see any folders you do not share. I found it much more efficient than FTP sites or email chains. Oh, there is also a web interface if you do not want to download the application.
Full disclosure: Anyone signing up for a free account through the link below receives an extra 250 Mb of storage space and I also get a bonus of 250 Mb. Trivial bonus aside, it really is a cool tool and I would be posting about it even if I received nothing.
Trask, The Last Tyromancer