Wifi as far as kilometers with… a coffee can
Wifi is the wireless standard, to connect PCs in small dimensions LANs, via radio.
The main problem is the distance: with some Euros is possible to buy USB keys that will give Wifi connectivity to our PC, but with a poor 15-30 meters coverage. It’s possible to make a little more with “access points” and good antennas, but what if we would like to connect to our friend that lives hundreds of meters from us?
I just discovered that with less than 10 Euros it’s possible to build an antenna with this coverage, that can teorically reach Kms, based on a simple coffee can.
Someone sells those antennas at 49.95$ 
The idea is basic as much as the can: a normal electric wire is already, because of its nature, an antenna; it’s problem is to be an ”omnidirectional” one, because the signal it sends goes all around, it expands itself and it is quicky lost. If we force it in a tube (our can), the signal is sent in a single direction, with a much powerful strenght. of course it’s necessary to use particular cables and connectors, and to make a correct study of proportions between the diameter of the can, the point where we put the cable in, and so on. Then, we will have only to give a standard “N” type connector to our antenna and to connect it to our Wifi board (or to our router, etc…) and to start trying to point it to the final destination ;)
Here is a selection of other sites with informations and step by step how-to’s to learn building “Cantennas”
TurnPoint
O’Reilly Wireless Devcenter
MyMac
PS: It seems that “cantennas” were recently declared illegal in some states. Make sure that law in your country allows you to build and use such kind of material.