Apparently around for quite a while but I just came across them recently. Slacker Radio is great for discovering new music, or at least music you missed but shouldn't of.

YouTube - Old Crow Medicine Show - I Hear Them All (http://www.youtube.com/watch?v=ug7IgB8MfWE)

YouTube - Old Crow Medicine Show - "Wagon Wheel (Live)" (http://www.youtube.com/watch?v=YH0CnjXqCLE)

YouTube - Old Crow Medicine Show - Down Home Girl (http://www.youtube.com/watch?v=cJhlM6W4uhk)

YouTube - Old Crow Medicine Show - "Caroline" [music video] (http://www.youtube.com/watch?v=uyUOe7HAkpk)

These guys are seriously good. As you said they have been around for a while with a good following. Staple long car trip tunes for me.

Found them a few years back while watching one of the country music channels. Also came across Cross Canadian Ragweed, country/southern rock sound to them.