Assuming you've got the standard tools for everyone working at the office - some sort of source control tool, some sort of project management bug/task tracking system, etc, the only other tool you're likely to need is an inter-office IM system - we use Skype, ourselves, and it allows us to keep in touch with our out-of-office employees, both for business reasons and to keep them feeling like they're part of the team by having them participate in our off-topic chatroom.
Beyond that, you need to stay in touch with your out-of-office worker, just like you'ld stay in touch with one working in the next room - chat with them at the beginning and end of the day to see how they're progressing on the tasks you've given them and to see if they're stuck on anything that somehow hasn't gotten to your attention. We also track the hours that people work on different projects, so at the end of a day we can get a rough idea of how hard folks are working on different things, and how long. That may be helpful as well.
Stay in touch with them, make it easy for them to stay in touch with you, and keep an eye out for any signs that things aren't going as well as you'ld expect. If you're the manager, it's the same, you just need to remember to stay in touch, since you're not going to see him in meetings and at the coffee urn.
Update: Someone commented with "What do you use to keep track of the hours someone has worked?" We're using Unfuddle at the moment. It's good enough for now, although it doesn't fit our needs as well as we'd like.