Google Trends
Google Trends is a service which allows you to compare the popularity of different search terms. The comparison is represented by using a line graph with “time” along the x-axis and “total number of searches” along the y-axis. Note that the graph shows time starting from year 2004.
The interesting part is that there are some search terms which are seasonal and others increase depending on some global factor. For example, here’s the graph generated by Google Trends for the term “Euro”. As you may notice, the graph has two spikes, one for Euro 2004 and the other for Euro 2008.

Just as another example, here’s the comparison of 4 different programming languages: C#, Java, C, PHP

You may try it out yourself here.
