While I have my own recipe for a mustard based sauce, when I don't feel like going to that much trouble (which is most of the time), lately I've been doctoring up Cattleman's mustard sauce - a bottle of Cattleman's; 1/3 cup molasses; 1/3 cup bourbon; 1/3 cup cider vinegar; a couple tablespoons of rub du jour (lately I really like the taste of the Rendezvous rub clone from the recipe at Amazingribs.com); and a couple tablespoons of worchestershire sauce. I mix it all in a saucepan and let it simmer for 15 to 30 minutes and adjust to taste. Comes out pretty decent IMO (and family likes it, so...)
Oh, and when I want to "kick it up" a notch, I add some ground Bhut Jolakia, which has a great smokey taste. I divide the batch when I do this since only me and my SIL can generally handle that much heat.