Most of the travel gear reviews on this blog are for travel items you take with you on a trip. But here's one that could relieve some frustration before you step out the door: a travel scale that tells you whether you are over the weight limit or not.
I actually received this Travelon luggage scale as a holiday present back in December and have used it a couple times since. My old method was to stand on the bathroom scale holding my luggage and see where it came out. Close enough when I'm well below the limit, but this is a far more accurate method if it's close.
You pop the hook out of the recessed holder on the back and put it around your suitcase handle. Turn on the unit, lift, and check out the verdict on the display. It needs AAA batteries, but they were included and I can't imagine they will run out of juice anytime soon.
On most commercial flights you can get away with 50 pounds for your bag, but on an African safari or Alaska puddle jumper you may be limited to 25 pounds. On a cheapo flight in Europe, the limit could be less than 20 pounds, or 8 kilos. If you have a rolling bag with wheels, you may be at 10 pounds before you even put anything in it, so this scale could tell you whether that fourth pair of shoes makes the cut or not.
There's another cool feature to this that also helps you determine whether you bag qualifies as a carry-on. There's a tape measure you can pull out of the side to measure the bag dimensions. Some of those rolling suitcases advertised as carry-on compliant are really pushing the envelope when it comes to total linear inches, so using this you can see if you are definitely in the clear or not.
I think my gifter got this for me from Target, but it's also available at Amazon for 20 bucks.