Assuming President Obama stays in office for the next 2.5 weeks, his presidency will be the 4th longest ever, by a single second.
While there have been many presidents who served 8 years (or 2922 days to be exact), since 1972 Presidents have begun spending just a little bit longer in office thanks to leap seconds. Leap seconds have been added to years to keep the Coordinated Universal Time (UTC) in sync with GMT time, and since the length of a Mean Solar Day has been getting longer because the Earth's rotation has been slowing down due to tidal friction.
Since 1972 there have been 27 leap seconds. During that time 4 presidents have served 2 full terms (counting Obama) and the leap seconds have not fallen evenly among them. Reagan and Clinton each had 5, lengthening their term by 5 seconds. GWBush had 2, and Obama has had 3. Meaning that behind FDR, Reagan and Clinton are tied with the 2nd longest terms, Obama is 4th, GWBush is 5th and the other 8 2-term presidents are tied for 6th.
So if it seems like Obama accomplished more than his predecessor, it may be because he had more time to do so.