There are lots of websites where you can gather the information .
archive.org: Archive.org is a website which is maintaining history of many websites over the internet. Many times you can get some information which is no more displayed on the website because of some security issue but something related to that can still be found there.
whois.com: In this website, you get the full information related to our domain.
netcraft.com: This website gives us a detailed information about the web hosting and the Server with detailed information on what is running on the server along with the IP, whoIs information, Server side technologies etc. All this Information should be saved in your reports so that you can use all the information to find the right tests and define the attack surface which is the most important part of a penetester.
yougetsignal: Many times the particular domain you are targetting is not so vulnerable or you are not able to find the right attack surface, in such case you can make a Reverse IP domain lookup and find the other domains on the server which may be vulnerable and allow you to enter the Server.
In such a way you can make your way towards into the website.