The closest google's getting me is this question here, but the answer (presumably "none, run a wire") is behind a paywall.
If you want one that plugs into your router use this one. http://www.radiolabs.com/products/wireless/wireless-range-extender.php
I have this one and it is awesome. Easily 2.5x longer range and stronger signal. We use it to send Wi-Fi about 1/4 mile down a trail at my camp.
This one goes on the device receiving the signal instead of sending http://www.radiolabs.com/products/wireless/802.11n-amplifier.php
I have no experience with this product, but I like the radio labs guys. Solid people there.
I use the one from linksys here. You can also get it at bestbuy. What router are you using though?