What are the most useful research sites you've found on the internet?

Research portals, general news and informations sites would be great – but if you have a particularly good site on a single topic, fire away.

If you know of sites that you’ve found worth paying a fee to use, that would be interesting to know.

Finally, why you think that it’s useful (e.g., evidence of its lack of bias, sheer volume of material, searchability, etc.) would be great.