1. Go to: http://www.routerpasswords.com/
2. Click the small black triangle symbol next to the blank space to select the manufacturer your router.
3. Then click "Find Password" button.
4. It will show you a list of model from the selected manufacturer together with the password on the right hand side.
Note:
If you cannot find the exact model of the router listed there, you can try a password from an alternative model from the same manufacturer.
Usually, the companies use the same or similar passwords across different models.