This topic is neat, and is something I have interest in. Here's a few I can think
broccoli leaves (and I think the whole stem too, but is rather fibrous)
cabbage leaves (people
seem to only want to use the head, those other giant leaves are good too)
I like radish, beet and carrot tops in my salad when I have them. The radish tops can take some getting used to, they're a little prickly.
I'm sure there are many others and I can't wait to see what other folks here say.