I first got on the internet in 1997. My brother told me how it worked, what hotlinks were and all that. The coolest site on the internet then and now is Amazon.com. No question about it. Almost any book you want to find is there along with reader comments - some unhelpful, some helpful, and some that are amazingly thoughtful and articulate.
The first thing I bought on the internet was a book from Amazon. It was Endurance by Alfred Lansing (about the Shackleton expedition - really outstanding). I was stationed in Camp Pendleton (California) at the time. I clicked the order button and within a couple of days, the book was on my doorstep. It was the coolest thing since canned beer. Still is.
You can buy a lot of other stuff on Amazon now, but books are my main interest. I think if Amazon went out of business it would probably tear a hole in the universe.
Amazon.com. The best.