I deposited and played at freebitcoin.com and after playing I was able to withdraw my bitcoins. I saw the transaction at blockr but don't remember which website I created that Bitcoin address on. Please help.
A bitcoin private key is a random 256 bit number. A bitcoin address is deterministically derived from that private key. Everyone uses the same procedure to do so. Therefore, it is impossible to figure out where you created the wallet based one a bitcoin address.
Your browser history, however, might lead you to the correct website. You can limit the websites to check to those you visited when you suspect to have created the wallet, therefore narrowing down the places to search.
Since you don't know the website and probably need credentials to access you wallet there, you probably saved the password in your browser. Since the number of saved passwords probably is much smaller than the number of websites you visited, searching your saved passwords might be the best way to go. Every modern browser should be able to show them to you, along with the website they belong to.