I would suggest you use Yammer (www.Yammer.com). It is what we use and it keeps people in touch. The other option could be Salesforce Chatter. But thats more for sales and BD folks.

I agree with Lucian about using IM type stuff to keep in touch - we have a lot of staff that are field based / work from home a couple of days a week and use Microsoft Communicator to keep in touch. But you can't beat f2f occasionally - once a week / month (depends how remote they are) to form proper team spirit

Why not ask the employees what will work for them - maybe even get one of them to take responsibility for organising any changes.

It really depends from the tasks... For collaborative writing google docs will help, real time communication can be held via a private IRC channel and I would suggest Powwownow for telephoneconference (my former employer) skype just sucks for more than 2 people conference.
In my opinion it is crucial to use the simplest technology available for each specific task. Try not to add unnecessary complexity or processes to your business. Transparency is the second most important thing I would care about when it comes to communication.

Technology definitely help, but at Newton we find that nothing beats getting together once a fortnight. We have a Thursday night social and a Friday project review to share best practice, company messages, problem solve as a team etc. Review days (as we call them) are absolutely vital to employee engagement and a core part of our culture. The Thursday socials are always good fun too (just need to watch out for hangovers on the Friday though!).

Thanks everyone. Like Ros and a few others have said. I'm not sure it's just technology that's the problem here. It's that physical difference and trying to keep people motivated, goal-orientated, without having that level of physical presence. Technology certainly helps, but there is plenty more to it.

I suspect as Ros says, we need to meet more frequently.

@Adam one of the fascinating things we've found is that asking employees what they want isn't always a good idea. Some don't see a problem for example. They don't see the picture from 'up high'. Have you had any experience here? Would be keen to know more.

@richard I have usually found that getting everyone together to discuss strategy and goals helps with motivation - it is often when empoyees don't understand this or feel uncertain that you lose focus. Which seems to be what you are describing. As a follow up question you might want to ask yourself if you are clear on your goals and strategies - as this is a definite precursor to communicating them. Whilst HR isn't my area (and others may jump in here) I wonder whether you have a good framework for appraisals and PDP's as having regular one on one conversations (maybe with 360 feed-back) may also help.