I refer of course to Wikipedia.
Eight years ago I suggested that Wikipedia would emerge as the most useful product of the internet, and I stand by my forecast. (Facebook? Twitter? Guardian's Comment Is Free? Ahem.)
Of course Wikipedia isn't perfect; it has some flaws, and sometimes needs to be taken with a pinch of salt. It suffers from occasional biases, depending on the topic — though it doesn't seem to me to have an overall bias in any particular direction.
Its open-editing model has worked extraordinarily well, something that would have been hard to predict twenty years ago. (I wouldn't have guessed it could, before seeing it in operation.) Founder Jimmy Wales deserves enormous credit for having had the vision to see the model through to fruition.
If Wikipedia had ads it would be worth billions, but it remains steadfastly non-profit.
Here is a link for making a donation to parent body Wikimedia:
https://payments.wikimedia.org/index.php?title=Special:PaypalExpressGateway&appeal=JimmyQuote&ffname=paypal_ec&recurring=¤cy=USD&amount=0&payment_method=paypal&uselang=en
Honourable mention: Internet Archive, which contains a ton of useful stuff and, like Wikipedia, provides its services to the public for free. At the time of posting this, an anonymous benefactor is matching donations to Internet Archive, dollar for dollar.