Maybe 5 years ago we could say "get a cyclocross (CX) bike" and be done with it. But what has happened is that CX bikes have evolved to be more and more specific to the sport, and consequently less comfortable (at least for me) for commuting 6 miles or more each way. I test rode a couple of current-gen CX bikes and my back was starting to ache already after just 15 min. of riding.

Adventure bikes have evolved from older CX bike designs to be something you can commute on, go ride all day on weekends, etc. as opposed to racing. That is why Jamis now puts their Renegade bike line under the Adventure category instead of Road or CX. The competitors to the Renegade include the Bianche Volpe (Classic and Disc), Kona Rove, GT Grade, Fuji Tread, Felt V85, and there may be others.

You mentioned triathlon. Triathlon bikes are yet another animal - you've got that weird looking handlebar and all that. My friends say they're uncomfortable for anything outside the sport, but I've seen one or two riders commuting on that type of bike.