The following sites keeps a database of the default username+passwords that are originaly used by the manufacturers of different hardware devices.
you can use it to find out how to login to an unknown router or wireless-AP you found. there's always the chance that the original password was replaced but you can usually to a reset procedure by pressing an external button for a few seconds to flush the user settings and restore factory-defaults.
note the security risk that comes from keeping the original passwords unchanged!!