For most people trying to install security surveillance cameras on their barn perhaps 1/2 a mile away, you can probably just hack an old Linksys router as a repeater to get the job done or even build a WiFi Cantenna.
But if you want to make a DIY long-range WiFi network like FabFi did in Afghanistan (hooking up literally every village together), you might just want to check out some of their open source instructions on how to do it yourself using old electronics/junk you can find around your home/office.
FabFi is an open-source, FabLab-grown system using common building materials and off-the-shelf electronics to transmit wireless ethernet signals across distances of up to several miles. With Fabfi, communities can build their own wireless networks to gain high-speed internet connectivity—thus enabling them to access online educational, medical, and other resources.
Project Summary (as of March 5, 2011)
Production Networks actively deployed in four locations across two countries
45 remote FabFi nodes are currently deployed in and around Jalalabad, Afghanistan
Longest link is 6,000m (3.72mi)
Data throughput 11.5Mbps
System extensible by anyone
Materials to make an endpoint link are $60US and available locally
50 remote FabFi nodes are currently deployed across three sites
Longest link is 3,500m
6-hop Data throughput across 2,500m, > 30Mbps
System provides WiFi direct to end-users
Sytem integrates user accounting and management