There are a few wireless range extenders on the market.
The Belkin Wireless Range Extender can be a bit tricky to set up, particularly if you use WPA. It is happier with WEP and happier still with no security at all. The supplied software only does half the job and you have to set a PC up with a fixed IP address wired to the unit to get to the full on-board config screens.
Sitecom 300N is pretty good, and supports Wirelss N, B and G. Antennae are also detachable so can be upgraded if necessary
Best value is the Edimax Wireless 300M. This supports N, B and G, has detachable antennae. It is also the simplest to set up. DABS.com knock these out for £40 or so.