Our house has dead spots which have been difficult to sort out and I have tried various techniques to get good access. Running cables around is difficult due to the two story construction and herself's, not unreasonable, objection to wires running in visible locations.
An early approach was the boost the power of the Linksys WRT54GL and put on higher gain antennas. This really didn't work very well although it seemed that it ought to. Quite a bit of experimentation with this and location failed to yeild any significant improvement.
Next up was the introduction of a second WRT54GL running through Netgear's Powerline connectors. I was utterly unable to introduce encryption onto these devices but they do seem to work well enough over modest distances. It's a cumbersome system but could work well enough in some circumstances.
A minor problem with the Netgear ethernet solution is the very bright blue light that the boxes emit and flashes when the box is being used. Not normally an issue but one room is a bedroom and the only choice was to tape over the LED.
Finally I cracked it, nothing novel and no real credit to me but you can get wireless repeaters, indeed the Linksys WRT54GL and many other devices can be configured as wireless repeaters using DD-WRT which is not simple to configure but you can buy wireless repeaters units off the shelf.
Put simply DD-WRT is able to join the WiFi network and then re-transmit, on the same frequency. This reduces the available speed of the network (by more than 50%) as it is using the same frequency to both transmit and receive however this is not a problem for us as the WiFi network speed still well exceeds our internet connection speed.
This is a great way to extend coverage either to dead spots or to adjacent buildings.
On a related note we have been able to introduce a 'guest' network using the Apple Airport Extreme. This network has a different password to our main network and prevents the guest from 'seeing' any of our computers/servers or whatever. Not that we don't trust our guests but there is a limit to how far you want your password to be known.