I've used cabbage, eggplant, and zucchini (alone and in combination).
I don't like large eggplant, so I only use the long, asian eggplants.
In my opinion, cabbage leaves best duplicates the lasagna texture. Zucchini I found too watery to use as the "noodle" substitute, but cut into matchsticks it makes a nice portion of the filling.
I've also made a lasagna type dish without any noodle substitute. I saute all sorts of yummy veggies (andy or all: onions, bell pepper, celery, carrot, eggplant, zucchini mushroom...) with ground meat or my ground meat/tvp mixture, along with homemade or jarred spaghetti sauce.
While that simmers, I make my egg/cheese mixture.
One egg, a large container of cottage cheese, ricotta (or a small container of each) some shredded asiago, parmesan or romano.
I line the lasagna pan with foil, pur in the cheese mixture. Top with the meat/veg/sauce mixutre and top with a little bit of shredded cheese (usually mozarella and asiago or a packaged italian blend).