Fernando Alonso, testing for Jaguar in 2002 at Silverstone.
Rick Mears testing in 80 for Brabham was with in a quarter of a second of Piquet and the story goes he was offered the job, but decided to stay at Penske in the U.S. to race.
From the Motorsport.com article I read, Rick Mears had two tests with Brabham over the course of 1980: One at Paul Ricard in France and the other at Riverside Raceway in California. At the end of the Paul Ricard test he was about half a second off of Piquet's time, but at the end of the Riverside test Mears was faster than Piquet.
Ultimately, though he and Brabham boss Bernie Ecclestone came to terms for a contract for 1981, Mears chose not to pursue it and stayed at Team Penske.
And here's Sage Karam/Marcus Ericsson.Marcus Ericsson testing for Brawn.