Robots.txt is a text file which is add by admin on the site to tell search robots which pages that admin would like them not to visit while.
so as you can see above , there are many secret pages are hidden from search engines .like :administrator,images,templates,tmp etc ...
so by typing www.websitename.com/administrator you will find the admin panel of that site !
now, let's see the 2nd way to find admin panel ,
so lets talk about cPanel , so cPanel is a graphical user interface to control the whole website.
to get into cpanel it's very Easy , just type www.websitename.com:2082
you can replace the port numbers after the ":"
Port 2082 Cpanel default port
Port 2083 Cpanel over SSL
Port 2086 Cpanel Webhost Manager (default)
Port 2087 Cpanel Webhost Manager (with https)
Port 2095 Cpanel Webmail
Port 2096 Cpanel secure webmail over SSL
okay, so now third way
download this admin finder tool From Here
so guys, these are my poor ways to what's your's share it . :)
and Enjoy .....