I'm going to go out on a limb and say I'm probably not the only one who does this...but I take a standard viper tap, push the plastick spring loaded bottle filler into the end (the one u used to fill bottles from the bottling bucket...but remove the spring loaded portion), and feed it through the center of a rubber stopper that fits the bottle opening. Hook it up to the keg and slowly "burp" the rubber stopper as it fills the bottle. Works like a charm. It does not "purge" the bottle of oxygen first, but I've never had a problem with the beers, even when left in the bottles for 4 months or more. Costs 10 bucks, if you dont own any of the items already.
I use a very simple homemade bottle filler. It consists of a picnic tap with about 3' of hose added downstream of the tap. This hose passes through a drilled stopper. Dead easy to make and dead easy to use.
The length of hose past the stopper is just less than the depth of the bottle so it fills from the bottom to minimize foaming. Also, I hold the stopper in place with light hand pressure, this further reduces foaming by maintaining pressure inside the bottle. When near full, remove the hose, and cap over the foam (just like the big boys do).
I use this just to draw off a liter or two into flip top bottles when I want to take some brew along to share. It works great for this.
If you really wanted to bottle from a keg, for sending to competition etc, then a more complex counter pressure filler might work out better.