I actually did [Google] but it just came up with the outer boxes for sale. I want to know what's in them!
A little tip: googling this phrase "Christmas Eve boxes mumsnet" for me brings up "What goes in your Christmas Eve box" and "Christmas Eve box, what's going in yours?" as the top two hits. I often do this when looking for a specifically MN thread on something, as it works better than MN's own search. I hope this is helpful 
I think it's fairly obvious though from "Surely there are enough presents given out on Christmas day itself!?" that this wasn't a simple fact-finding mission, more that you wanted to TELL OFF PEOPLE WHO DO THIS. 
For the record, I've done it since my eldest was old enough to understand (40 years ago) and we would give her one present with our 'special' Christmas Eve dinner, a small one waiting at her place. Not an extra present. In later years I realised that any themed Christmas clothing like knickers, socks and giant Christmas-themed jigsaws were best handed out the day BEFORE Christmas. We still do this.
Several people this and every other time this topic comes up excuse their urge to Tell People Off with pious concern about 'pressure on parents'. People can say till they're exhausted 'it's hot chocolate and a DVD to share on Xmas Eve' or 'it's new clothes for Xmas Day' but the urge to tell people you disapprove, don't like it, don't think we should do it, must be really strong because - there's two weeks till Christmas - plenty of time for more wide-eyed innocents to post the question fifteen more times and for the piously concerned to express their virtuousness.
Oh and, "I do find greed a bit unappealing" wins the thread.