I posted earlier a story about simple USB-based WiFi amplifier. Many yachtsmen use instead Ubiquity NanoStation and it has been proven working quite well.
Basically, it’s WiFi-to-Ethernet router. It receives WiFi signal via 4 internal antennas, amplifies signal, converts it to copper and passes to Your computer via ethernet cable. All electronics is inside the 26 cm long antenna, so there are no signal losses and noises compared with products using long antenna cables. NanoStation uses power over Ethernet method to provide required power to the circuitry, only single CAT5 RJ-45 cable is needed for connection. Additional RJ-45 POE hub is included.
NanoStation works in b/g range, so don’t expect too swift data transfers, but still much more reliable connections, than std WiFi. NanoStation 2 is directional, so You have to align it towards to base station. Expect to pay for NanoStation abpout U$ 80. Highly affordable product!
There are real-life reports about reliable WiFi at more than 2 miles distance.