As long as what is in the jar can stand it....run the hot water in the sink. Let it get really hot. Stick the metal lid under the hot water for about 30 seconds. The hot water will cause the metal to expand slightly. The lid will come off easier. This won't work for plastic lids.
They often give them away free (with advertising printed on them) at the county fair where I grew up. I tell my parents to grab a few each year, because a lot of my friends want one after they've tried it.