The major options that are widely in use, as far as I know them, are: Avid ISIS/Unity, Facilis Terrablock, EditShare. Then I was sort of looking into alternatives like Dynamic Drive Pool, but haven’t tested any of them. You also may want to check out Strawberry as an add on for any of them (shared project management software).
From what I have been told, if you want to share the media, you’re sort of OK with all of them, but if you want true, easy, transparent project / bin sharing, you need Avid’s systems. They are completely built for several people working within the same project at the same time. For all others, you can sort of get the same result, but you need everyone to work very disciplined.