Delta's weight limit is 50 lbs., Jet Blue is 70 lbs. I use both of these airlines often and it depends on the porter/ticket agent as to how strict they if you are slightly over (a few pounds at most)
I believe all the rest of the major carriers are also 50 lbs. Once you find out what airline you are going on, I would either call them or check their website. It is pretty expensive if you go over the weight limit.