Kottonmouth Kings are perhaps the biggest "marijuana music group" out there today. They are all about marijuana all the time. Even the band's name is named after a popular marijuana side effect. "Cotton Mouth" occurs after you smoke, it dries out the smokers mouth for hours, unless you get something to drink. This song, "Where's the Weed at" is a hit on youtube with well over 2 million views.
One of the great things about musicians in the underground weed culture is their dedication to their fans. Kottonmouth Kings are always doing something for their fans. They release new videos and content regualrly on their blog, KottonmouthKings.com. They also do plenty of interviews and other press media. Check 'em out, you'll probably find some bumping new tunes to blaze to.