Here is one of many adapters available:
It attaches to the camera just as a Micro Four Thirds lens. Here is a video showing how to attach the adapter to the Panasonic Lumix GH1.
It reads "NIK-M4/3", and can be used to mount Nikon F mount lenses to a Micro Four Thirds camera. These kind of adapters are simple, meaning that they only provide a means to mount the lens, and no control over the aperture or focus is possible from the camera. On the positive side, they are rather inexpensive, and can be purchased for around US$30-40.
When using a third party adapter like this, the camera has no electronic confirmation that a lens is actually connected. For the camera to still operate, you need to set the menu item "shoot without lens". This is found under the "Custom" menu, indicated by a "C" with a wrench icon. Here is the menu item from the GH2 camera:
In the picture below, the adapter is used to mount a Nikkor 200mm f/4 AIS tele lens to the Panasonic Lumix GH1. When using this legacy lens on a Four Thirds camera, the field of view becomes equivalent to a 400mm lens. With a long tele like this, using a tripod is a necessity. While you can choose fast shutter speeds and capture an image without blurring due to camera shake, framing the subject is very difficult when handholding this combination. Recording video without a tripod is virtually impossible with such a long lens.
This lens has an aperture ring. Since the camera cannot control the aperture, it needs to be set manually. The presence of an aperture ring makes this operation easier. Some more expensive adapters have a means for controlling the aperture also for lenses without the aperture ring, e.g., the Nikon G mount lenses.
Many newer lenses do not have aperture rings, and require an adapters with a lever to stop the lens down. This goes for both Nikon and Pentax lenses.
There is some talk on the internet about adapters that provide focus confirmation for legacy lenses. To my knowledge, such a thing is not possible with Micro Four Thirds cameras. The cameras cannot confirm the focus when using non-compatible lenses. In fact, with the current cameras, the only way for the camera to know that the image is in focus, is to use a Micro Four Thirds lens, or one of the CDAF compatible Four Thirds lenses.
To be able to confirm the focus, the camera must jog the focus back and forth to find the optimum contrast. This is not possible with manual focus lenses, of course, since the camera cannot control the focus at all. So focus confirm adapters for Micro Four Thirds simply make no sense at all.
A fun fact is that the adapter is larger than the Panasonic Lumix 20mm f/1.7 pancake lens. (The lens to the right below.)
Here are two example pictures taken at f/4:
And an example video:
I have also looked into an adapter for Pentax K lenses.