I’ve been a fan of the Linksys WRT54 series of wireless routers for quite a while now. They are cheap, and you can replace the firmware with the excellent DD-WRT or Openwrt, which are both GPL’d GNU/Linux distributions for the platform.
There are many versions of the WRT54 routers. The wikipedia page on WRT54G has a comprehensive overview. As things stand currently, the WRT54GL is the best choice if you want to install a Free operating system onto your router – which is highly recommended. The GL is the version Linksys has put out specifically for the purpose of supporting 3rd party firmware.
The ‘list price’ for the GL is about $60 to $70. However, because of a rebate and a Google checkout discount, it can now be had for $36.55, with free shipping, from Buy.com. Highly recommended.