iMac wireless signal strength low

In Leopard the reported signal strength is not just a matter of pure strength but of quality as well.

You may want to download iStumbler and check to make sure that no new Wi-Fi routers are now conflicting with yours.

You're missing my point.

If say, your router uses Wi-Fi channel 6, and a neighbor decides one day to buy a router and his also uses channel 6, you will see a reduction in signal level on your iMac due to the interference on the same frequency from your neighbor's router.

This same thing can happen if your neighbor one day buys a 2.4 GHz cordless phone.

Of course if your nearest neighbor is a mile away, this isn't an issue but since you mention you can see multiple networks, it's good to check to make sure there aren't any new ones that could be interfering with the signal and/or that none of the other networks are now using the same channel as your router.

The only Wi-Fi channels that don't interfere with others are 1, 6 and 11, so in general you should pick one of those three if it's unused or the one with the weakest existing signal if you can't.

BTW, as a result of this, in many more crowded metropolitan areas, the bands are getting so crowded Wi-Fi just doesn't work any more unless a user has 802.11n equipment only, allowing them to move to the less crowded 5 GHz band…