When I'm training for a long run, like a marathon, I like to carry a 1 lb dumbbell in each hand for my week-day short runs.

Maybe I'm wrong, but I feel like they help prepare for some of the fatigue that can come from a long run. And, in particular, I get sore between my shoulder blades on really long runs - but I feel like running with small dumbbells during my mid-week runs helps build and condition those muscles for the long runs.

Aside from that - yeah...not many uses for a 1 lb dumbbell.