Google Trends

I remember this tool every now and again, and am reminded of its usefulness. Google Trends displays the frequency of searches for particular terms on Google.
These go back to 2004, so there are now around ten years worth of data to use, so it is good for assessing recent events and trends.
Here's the global searches for the term 'Eyjafjallajokull' showing little interest in the volcano until March 2010.

One useful additional feature (as shown above) is that the trend graphs that have been created so that they can be embedded into webpages.

Useful to be able to search by country, time period, imagery etc.

Add more than one search term to compare the 'interest' in those terms over time...