Actually, it's a little candle holder, and instead of marbles, I use those little foam letters (because I have literally thousands leftover after Jessie's birthday party). I read this idea somewhere, so I can't take total credit - but I started this new policy. Whenever I notice my kids working together, playing together nicely, being kind to one another, etc - they earn a letter in the jar. And when the little candle holder fills all the way up - they get some sort of treat (to be determined later).
Since instituting this - I've noticed that they actually are nicer to one another. Granted, Sam has twice already asked for letters for playing quietly by himself, and today, Jessie begged me to please just get rid of Sam because she just couldn't take living with him anymore - so it's not perfect. But I'm pleased with the experiment - and it's well worth the price of the ice cream cone that I'll buy them at Friendly's once they finally fill up that little cup.