My day job as a WiFi installer has shown me that 10km is very doable. Using the proper gear helps and seven or eight years ago we did experiment with Yagi style Cantenna. These are very directional and difficult to keep aimed in the wind. For the cost a grill is better, Check out: http://www.pacwireless.com/products/point-to-point.shtml
Depending on the frequency being used a old satellite dish works well too. I cut off the LNB and braze a mast to hold a 5.2ghz Motorola Canopy
Looks like this this increases the gain by 18dbi, enough to triple the antenna power. We routinely install Dish mods to increase distance to 10 miles and more, if line of sight is there.
There is a new kid on the block that might be worth considering Nano Station 2 by ubiquity this manufacturer says 15 kilometers is easy, but I haven’t seen it
Now this deal of using the old Linksys routers to flash with Linux is way cool,
we have turned a $5.00 used router into a highly configurable access point comparable to a Cisco worth many thousands of dollars which unfortunately when Cisco bought Linksys they immediately reduced the flashable ram down to less than a meg to stop people from making these routers open source friendly. I am still waiting to find one on Ebay and have too many other irons in the fire at the moment. When flashed with Linux the Linksys router can be set to have different power levels on either antenna and use one for transmit and one for receive. Too many options for me to remember.
Linksys WRT54G series
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Linksys WRT54G version 1.0
Linksys WRT54G (and variants WRT54GS, WRT54GL, and WRTSL54GS) is a Wi-Fi capable residential gateway from Linksys. The device is capable of sharing Internet connections among several computers via 802.3 Ethernet and 802.11b/g wireless data links.
The WRT54G is notable for being the first consumer-level network device that had its firmware source code released to satisfy the obligations of the GNU GPL. This allows programmers to modify the firmware to change or add functionality to the device. Several third-party firmware projects provide the public with enhanced firmware for the WRT54G. See Third party firmware projects.