In short: Only use channels 1, 6, 11 (and 14 if you live in Japan) for your 2.4 GHz wireless LAN and simply ignore the other ones. The world will be a better place with no effort at all 
Great! Now you have got a WiFi of your own. As you are a prudent guy you’ll carefully choose a free channel so as not to interfere with your neighbors’ WiFis. Scanning for available networks reveals that channels 1, 6, 9 and 11 are already being used, which leaves channel 3 as the way to go. You are quite confident that everything should be in order now. The truth however is rather disconcerting and may come to you as a shock.
What you have really done is, you have set up a new jammer that will halve the throughput of at least 3 networks - including yours. How can that be true?
Only 3 out of 11 channels do not interfere: 1, 6 and 11
This is due to the channel allocation scheme that is used by 802.11 b/g standards and the behaviour of WiFi equipment: While channels are equally spaced 5 MHz apart, the bandwith consumed by a single transmission path is roughly 20 MHz, effectively occupying 5 channel slots: The center channel it is set to, 2 adjacent channels towards lower frequencies and 2 channels towards higher frequencies as well.
Thus a station set to use channel 1 will occupy channels 1-2-3. The next non-overlapping channel will be 6 occupying channel slots 4-5-6-7-8. The only channel left will be channel 11 occupying channel slots 9-10-11.
Overlapping channels substantially decrease throughput
Assuming urban surroundings and just 3 useable non-overlapping channels, wireless networks are bound to interfere with each other. If interference cannot be avoided, why not try to minimize interference by “filling the gaps”? Two stations using the same channel should most certainly experience much more interference than if they were using different ones, even if they were just one slot apart. Suprisingly enough, this is not true.
Wireless stations are by no means dumb passive devices. Before initiating a transmission they will listen for other ongoing transmissions and abide from starting their own transmission when they find that the channel is currently in use, thus minimizing interference (DCF, CSMA) . Sadly enough, this is an important factor that is often overlooked or set aside, even in academic documents.
Now when you offset two stations by less then 5 channels you’ll impede their capability to properly detect ongoing transmissions - practically blindfolding them and turning them into anti-social criminals.
Furthermore, by virtue of its bandwidth, each transmission that does not adhere to the 1-6-11 rule will use up 2 instead of 1 out of the proper channel bands described. Thus stations set to channels other than 1, 6 or 11 will unknowingly and unneccessarily interfere and disrupt both each other and other adjacent channels, in turn causing frequent re-transmission which will bring throughput down to a crawl.
What about channels 12, 13 and 14
Within Europe and Japan, there are two additional channels available: 12 and 13. But they are of little use, as they both overlap with channel 11. Still worse, devices may refuse to connect to those channels, unless they have been set up properly, e.g. a Nokia E6x will no longer connect to channels 12 and 13 when you remove your SIMM.
Channel 14 is a very special case. It is used in Japan only and despite its number it is spaced 12 MHz apart from channel 13, so it might have been named channel 15.4. Transmissions on channel 14 will interfere only minimally with those on channel 11. Therefore, in Japan there are 4 channels that should be used in order to minimize interference: 1, 6, 11 and 14.
Links:
- Channel Deployment Issues for 2.4-GHz 802.11 WLANs (Cisco), as PDF
- 802.11b WiFi Channels (Moonblink)
- DCF (Wikipedia)
- DCF (WiFi Planet)
- CSMA (Wikipadia)
- 802.11 (Wikipedia)
Comming next: Why SuperG and other 108 MBit transmissions often cause more damage than benefit. Why a 3 dB Antenna boosts more than increasing Transmit Power by 12 dB.