Google is testing using Roboto font in the search results. Conrad O'Connell spotted this test and posted it on Twitter.
Here is an animated picture of the normal font and the Roboto font so you can see the difference:
The Roboto Font was created by Google's Christian Robertson and revealed at Google I/O in 2013. It may be that Google want to switch to this font, because it is pretty crisp and clear.
The Google font description is:
Roboto has a dual nature. It has a mechanical skeleton and the forms are largely geometric. At the same time, the font features friendly and open curves. While some grotesks distort their letterforms to force a rigid rhythm, Roboto doesn’t compromise, allowing letters to be settled into their natural width. This makes for a more natural reading rhythm more commonly found in humanist and serif types.
Google also added that option to AdSense back in 2013.
Forum discussion at Twitter.