I'm not sure when the whole craze started, but Hershey's is issuing lots of limited edition candies. After a while, they get pulled of shelves, in some cases never to return (thus distinguishing them from the McRib, which seems to come every two years or so).
Some of these limited editions are gimmicks, like the black and white M&M's, but I strongly recommend that you try and get your hands on the new limited edition KitKat dark chocolate bar. I saw them at a gas station this past weekend when Bill and I were driving out to Tiger Mountain for some mountain biking, and I stashed one away as a post-ride reward. I only found it in my Camelbak today when unpacking my bike in Chicago. The dark chocolate limited edition is much better than the white chocolate version of which I also purchased one. The dark chocolate adds a certain je ne sais quoi. Oh, it's so naughty.
You can also stock away cases from sellers on eBay to tide you over once the limited edition sells out.
Are the limited edition Inside Out Reese's Peanut Butter Cups still out? Maybe I should snag a case off of eBay. I never did get my hands on those.