Since you are talking about work, there are two or three options for most companies, depending on what you already have or are willing to buy. If you run LotusNotes Domino servers, they can setup QuickPlace if it doesn't already exist. The MS alternative is SharePoint. If you are looking for a more freeform package for a small group, you can also look at MS Groove. I am sure there are many other alternatives, especially OSS alternatives, but I don't know any and even if I did, I couldn't tell you how good they are.
My experience is with QuickPlace and SharePoint. I am not so fond of either, they just both seem very cumbersome. However, they are capable of doing (more or less) what you are asking for. Groove is new with Office 2007, but it looks promising if you want to try that out. I have it, but have never had the need or opportunity to use it. Microsoft offers a free trial of this as well to see if it would work for you. It looks like it is good for 12 months, so it is worth checking out if you are seriously looking at it. If you do, let me know what you think.