I’ve got my grandmother’s.
I never take it to office parties, though.
I don’t. I transport the items in question in a flat plastic pan with crumpled plastic wrap between the units. Less mess, and the pan isn’t a uni-tasker.
Rum cake. The base is packaged yellow cake mix. You need a bundt pan. Easy to make and everybody will love it. You can get it on the Bacardi Rum website, I believe.
Another popular item for potlucks seems to be deviled eggs. I rather thought they would have been mentioned already.
My coworker makes this salad every year for our office potluck:
Broccoli cashew salad
Mix together Broccoli, shredded cheddar cheese, cashews and cranberries. Make a dressing out of mayo and sugar. (Go easy on the dressing, is doesn’t take much.) This salad is yummy and easy. Plus you can buy the broccoli already separated into flowerettes and the cheese already shredded.
I like to take homemade fudge and toffee. The toffee just takes butter (not margarine) sugar and salt and can be made in the microwave. Also good are homemade lemon bars. For the holidays, I make lime bars instead.
Spinich Dip
1 pkg frozen chopped spinich
1 pkg dried vegitable soup mix
1 (small) can sliced water chestnuts
1 cup mayo
2 cups sour cream
Mix sour cream, mayo and soup mix in a bowl, put in the fridge. Let spinich thaw, then squeeze as much water out of it as possible. Chop up the water chestnuts into little bits. Add spinich and water chestnuts to sour cream. Let flavours mingle overnight in the fridge. Serve with bread (its wonderful on pumpernickel) or your favourite cracker, or even veggies. (no need to heat)
I usually bring baked ziti to our company pot lucks, but that’s because I bought one of those catering sets at the local party store last year and I need an excuse to use it. I cook the pasta and mix it up in the pans the night before, pop the pans in the oven while I’m getting ready for work in the morning, and bring them to the office in a hot/cold thermal bag. The food stays hot until I set up the tray and the sterno.
Of course, you said you can’t bring anything that needs to be heated … go with the deviled eggs. 
I’m a freak. I don’t like deviled eggs.
I usually either make strawberry daiquiri pie or chocolate eclair cake, or I buy a vegetable tray.
Or you could make marshmallow snowmen, which I just invented tonight. 3 full-size marshmallows, stuck together with vanilla frosting. Use more vanilla frosting to stick on M & M Mini’s eyes, mouth and buttons. Stick in toothpicks for arms.
I feel like I’m going to vomit after reading the ingredients of most of the recipes in this thread.
Buy a ripe pineapple and slice it up.
Well, you all had wonderful suggestions, but I’ve decided to go out on a limb with my own concoction of egg, hard-boiled and sliced in half, with the yolk mixed together with mayonaise, mustard, diced and sauted mushrooms, and roasted garlic, all poured into the “cup” of the 1/2 egg.
If only I had a name for it . . .
They sound heavenly. How about “Angel Eggs”? 
Yum, they sound eggs-cellent!