In North America, one should always use either channel 1, 6 or 11 because all of the other channels overlap the frequencies of adjacent channels. In other words, using channel 4 interferes with everyone nearby using channels 1 AND 6! If you live in Japan, you may also use channel 14, but if you live in some other countries, it may not be legal to use any channel other than 11.
Here’s a more complete chart of the channels and how they overlap. 1-11 are all that are allowed in the USA, 12-13 are also used in EU, and 14 is also used in Japan. Looking at the chart, notice that Ch 14 is not offset the same way as the other channels. This is not done just for illustration purposes here. Ch 14 starts on the same frequency that Ch 11 ends on, without the usual gap of 5 mhz, so that in Japan one can choose from FOUR different channels that (relatively) do not interfere with each other. 8)
The maximum number of available channels for wi-fi enabled devices are 13 for Europe, 11 for North America and 14 for Japan. In North America, only channels 1, 6, and 11 are deployed for 802.11b/g. http://en.wikipedia.org/wiki/Wi-Fi#Channels
The following chart indicates which channels may be used in a variety of countries:
Please note that this chart may no longer be accurate. It has been stated that Spain’s restrictions are much less now, however their terminology does not specifically address wifi channels and needs some interpretation. Please consult these resorces if you are interested: