When you are out-and-about doing networking jobs it is quite likely that you will need to login to the clients router to change some settings. However, they forgot their password or never knew it. You ask them for the manual and of course they dont know where that is either. This list has the default login passwords for most well known router manufacturers.

Although this list is a little outdated. Most manufacturers keep the same passwords for all their products so if you cannot find your product, find something similar by the same manufacturer and give that password a shot.

http://www.phenoelit.de/dpl/dpl.html