I noticed this sensor hanging out in the engine bay when I replaced my MAF today. Where is this guy supposed to plug in and what is it? Thanks!

 

That is the intake air temperature sensor. It plugs into the air filter box on the side towards the firewall. Look for the opening and plug it in.

 

+1 as Dennis has stated.

But ... I can't tell from the looking at the components in the background of your photo what year your car is.

I know that in a '99 and I think in a '98 the IAT inserts into the elbow of the air intake snorkel (#14 in the illustration).