For my purposes, a sport touring bike combines the best of both worlds (sport riding and touring). Sport touring bikes were very popular during the bike boom of the 1970s-80s, particularly among Japanese models. However, a few bike companies are still producing fine sport touring frames -- including the Gunnar Sport, Soma ES and San Marcos, and the former Salsa Casseroll. Many cross bikes also come close to fulfilling the same niche, including the Gunnar Crosshairs, Soma Double Cross and others.

I've got two sport touring bikes that I use regularly for commuting and light touring, a Gunnar Sport and a Waterford RST-22, and I recently sold my Salsa Casseroll. I used the money from the Casseroll to buy a Ritchey Breakaway Cross, which is a combination travel and cyclocross bike. I have no trouble keeping up with group rides on any of these bikes and have ridden them all on supported tours. I also have a Bob Jackson World Tour, which is a full-on touring bike. It can handle much larger loads, but is slower and less fun to ride than my other bikes for normal riding.