You're looking for a collaboration tool. Google Wave, before it was shuttered, sounds like exactly what you're looking for.
There are several open source collaboration tools (think, free versions of Microsoft Sharepoint) and you'll need to decide what features you want.
- Should it be local or hosted? Do you want to have to manage, support, and update it yourself or can you deal with it being hosted elsewhere on the internet?
- If you're going to run it locally, what sort of system can you host it on? What OS? What sort of power does the host system have?
- If you're having it hosted elsewhere or using a free web-tool, what sort of access do you have to it? Does it have local apps you install? Does it have mobile app versions if your team uses smart phones or tablets?
- What is the target use? Is it to be used for project management? Real-time collaboration? Generic file sharing? Note-taking/mind-mapping/brain-storming?
- Does it have to be able to host files internally or can it merely reference files using links?
- Who has to own the data? Is being able to move the data easily between various solutions important?
Answering these questions will allow you to pin down what sort of tool you actually need.
Based on this being a small team, it's more likely you'll be looking for a tool that's hosted elsewhere. A remotely hosted tool will also allow for more easy access by your remotely located team members. In fact, I'd completely avoid a self-hosted machine unless you just really have to get into that level of management.
So then you need to decide if you want a Wiki system, or a more proprietary system. Many web-based collaboration systems are free for small teams and with limited features.
Wikipedia has a list of Collaborative Software that includes web-based tools, but only includes a few hosted solutions. It also includes a feature metric that could be very handy for feature comparison: http://en.wikipedia.org/wiki/List_of_collaborative_software
The last 2-3 years have seen quite a growth in the market of hosted collaboration platforms, so the good news is there's lots to choose from. The bad news is that this can make it hard to choose between offerings.