Rudy Project SPORTS DUFFEL

This bag is great, made of super sturdy material that has survived use as a tri-bag, cycling bag, and flight check-in luggage. Like others have said, it's big enough to fit your gear (all my tri stuff fits great: pool towel, running shoes, cycling shoes, helmet, water bottles, clothes, etc). 
If you need a bag, get this one, you won't regret it.