A color enlarger head. These were a real rarity when I learned to print.
I bought a condenser head with incandescent light for my Omega D2 planning to use a filter drawer to start printing in color, and literally before it arrived, found a D5 with color head on sale, near enough the shipping wasn't going to break me. The variable condenser provides a nice place for a filter drawer, so I'd have managed to get into color printing with that, but it's so much more convenient when I can just spin the wheels to set the color filtration. Makes multigrade and split-grade easy, too. And now I have a D5, with cold light and variable condenser head, that I can sell off to someone more or less local at a good price.
Well, sure, having two enlargers would be a luxury, too, but I don't think I'd use the condenser head enough to justify keeping the second machine, and anything I could do with the cold light, I can do with the color head, easier.