Coworkers at my office frequently bake (or otherwise prepare) various foods and bring them in for everyone to share. I usually don't partake in that, as I'm afraid of what some of the stuff might be laced with.
When we have part time/temps working long or unusual hours for us, a coworker and I have been known to buy pizza or subs for everyone's lunch or supper.
But don't get me started on the required participation in the birthday parties, where someone goes and buys a cake for everyone in the office to share while we waste hours and hours of time "partying". Three things about that annoy me... everyone in the dept is required to attend the birthday parties, they invoice everyone in the department for the distributed cost of the birthday cake and card, and then waste tremendous amounts of time sitting around. If I liked most of my coworkers and had nothing better to do, then fine. The money they invoice us for isn't the issue, it's the idea that I have to pay when I would MUCH rather not waste my time with the crap.