Here are my "definitive sites" which make the internet the amazing thing that it is ...
Google - the surface area of the earth is, tracked down with the assistance of Google, about 200 million square miles. Imagine trying to find something within one of those square miles. You need a map - a good one. The internet has 8000 million pages. You need a map - and Google is the best one I have found.
Amazon - this has saved hours of shopping, miles of walking. There are other shops, and you can't get everything from Amazon. But you can get a lot. The only problem: will it be the right colour in real life?!
The BBC - the definitive media organisation. Yeah, £120 is a lot to have a TV - but to be quite honest, the BBC is so good, I'd be inclined to pay them that even if I didn't have a TV. Reliable, informative, innovative. There is nothing like it anywhere else in the world.
Wikipedia - an open-source encyclopaedia. This will become the definitive repository of knowledge for the human race within the next ten years - more flexible, more updatable, more provocative than any printed version. If it could be compressed onto a PDA, it would be the Hitchhikers' Guide to the Earth.
Blogger - does for opinions what Wikipedia does for knowledge - democratises communication - at least until Google becomes more "useful" - i.e. more specific. Probably, by discouraging people whining at home, it has done more to stabilise relationships than anything since marriage guidance counselling.
And the almost-definitives?
ebay - brilliant, but just too flawed - too self-absolving.
There must be more. I'll have a think.