The TP Link WR703N is a tiny little embedded Linux system, sold in China as a travel 3G router, you can pick them up for less than £20 from eBay. They are very popular with hardware hackers as they are cheap and flexible. If you look carefully on eBay you can find a modified version of the WR703N which has 64MB of RAM and 16MB of flash as well as having it’s serial lines connected to the micro usb port, meaning you get serial console access over the same cable as you plug in for power! These versions are made by a guy in China with the handle SLboat, he has documented the process on his wiki or you can buy them ready made for about £30 including postage.
Out of the box unmodified devices run a Chinese language firmware but you can easily wipe it and install OpenWRT, mouse over the links until you find the firmware upgrade link which is http://192.168.1.1/userRpm/SoftwareUpgradeRpm.htm. Once you have found the link upload the OpenWRT factory upgrade firmware from the Attitude Adjustment RC1 branch wait and few minutes and then do basic OpenWRT setup. If you need help OpenWRT wiki is useful, once you have done the basics you now have a tiny low power wireless Linux box.