The Cantenna is a higher gain, directional Wifi antenna that one can construct out of the right shaped empty tin can with some high quality connectors and cables.
To use this you need to have a PCMCIA Wifi card that has an antenna connection, like the ORiNOCO or Buffalo cards.
You can find more information on constructing your own here
I purchased mine off the internet for about $20 complete with tripod and cable from eBay.
The cantenna certainly does seem to make a difference. My connection with the cantenna stayed up while others around me failed.
The cantenna is directional which means you have to point it directly at the source of WiFi access which makes it a little difficult to set up and a problem when swinging at anchor. If you are going to be stationary in a marina for a while, it is definite advantage especially when you are competing for bandwidth among other cruisers using the system, and if there are other WiFi systems on nearby frequencies conflicting.
I think that the biggest advantage of the Cantenna is that it cuts out interfering signals coming from other networks and boats around the marina.Howard [ Just Imagine ] 22-Mar-2008