I've encounter customer forget their router password when I've got to change their old one with new one after they upgraded their Internet Line.
Then I need to recovery/reset that password because of I need to check current router configuration to configure new router.
OK...below is the steps to recovery/reset the cisco router,
Connect your laptop and router by using console port.
Boot the router and interrupt the boot sequence as soon as text appears on the screen by pressing Ctrl and Break keys together.
Change the configuration register to ignore contents of NVRAM by keyin "confreg 0x2142"
Reload the router by keyin "reset"
Enter privileged mode by keyin "enable"
Copy startup-config into running-config by keyin "copy start run"
Change the password by keyin configure terminal, "username" "password" and "enable secret new password" (put new password that you want to reset in "password" and "new password" places.)
Reset configuration register back to default value by keyin "config-register 0x2102"
Save the configuration by key in "copy run start"
Now, recovery/reset the router password is successfully done.
Although the above steps are for 1700 and 2600 series router, I've used on other some router and still can use.
If you want to know in more detail for this, please go through cisco website and check.
Here I've take sample screen-shots done with Cisco 1841 router for your reference.
(Be knowledgeable, pass it on then)