These are 4 mm long lemon ants (citronella ants) from Maryland. They're farming honeydew from mealybugs, which are the little white insects some are carrying. In this case they were busy relocating the mealybugs to safer grounds because I had found them. View large to really get the full effect.
From the guys at bugguide.net: Acanthomyops has been moved into Lasius and in North America is called the claviger group. So this is a Lasius, but I don't have a keen enough eye to say which group they belong to. Most likely it's the umbratus or claviger group though, but I'm not certain it isn't in flavus either without a better close up of the head and palp under it." - MrILoveTheAnts
"Turns out both names are right, since Acanthomyops was place d (back) into Lasius a few years ago. As for the species, can't really see enough detail. If truly 4mm long, then the only one it could be is L. (A.) intterjectus, but these look, in a "Gestaltisch" way, more like the 3-3.5mm murphyi or claviger to me." - James C. Trager
This photo was also featured by the Science Channel here, as the 6th reason to respect the ant:
I also have another photo here: