I have a couple.
In a previous fort, I decided to build a couple of tunnel-access-only Marksdwarf bunkers behind my moat to expand my defenses a bit. Then I decided to add a couple more. Then after that, I decided to connect them with a fortification-lined bridge. Then I expanded it a bit more.
Like any great drunken engineering project, it wound up as a monstrosity with a mind of its own that I was powerless to stop. Fortification-lined Doom Bridges hundreds of tiles long, half a dozen bunkers, a hanging ballista platform, direct links to the barracks and ammo stockpiles, and the beginnings of a Doom Bridge that would extend ten screens above the caravans' usual route. And it was color-coded by Z-level, to boot.
Then 181.39 was released, and I decided to start a new one rather than risk corrupting it.
The new one has a building project that is not really a defensive monstrosity so much as it is a piece of landscape management of which I'm actually rather proud. It's a 3-wide bridge extending one arm to each edge of the map for traders to use. It even goes over some rather vital areas of the fortress, and is largely lined by walls (to keep attackers from shooting caravans atop it and from shooting down into aforementioned vital areas).
The best part, though, is that the map is rather conducive to cutting off caravan access, with heavy forestation and a river going north-south that cuts the map almost in half. Some removed ramps here, a strategically-placed statue erected in the woods there, a few moats that would be used to cut off secure lumber farms anyways, and suddenly, the bridge is the ONLY way for the Human and Dwarf caravans to get in.
One idea I've had that I haven't gotten around to building yet is a "Paratrooper Pod". It would be a platform sticking out over a typical siege route, completely surrounded by walls, and held up by a single support...with a lever attached. A siege comes, you put some melee soldiers on the platform, and wait for the goblins to be directly underneath. Then, pull the lever.
The platform collapses, the goblins have a bunch of rock fall on them, and when they come to, they're surrounded by bearded drunks with very sharp objects. In theory, the dwarves would have thick armor and a LOT of levels of toughness, so they would come to quickly and more or less unhurt. In theory.