Most airlines are around 50 pounds per piece, BUT the best thing to do is check with the airline you are flying on. You can look it up on the airline's website or just give them a call. If you know you'll be overweight no matter what, you can also find out what their overweight bag fee is, usually it's around $25.
It used to be if you were only a pound or two over they'd let it slide, but I've heard they are much more strict these days about enforcing it, so be careful what and how you pack if you are determined to avoid the fee.