I have more!
Hotel, the sign is lit up at night via a light sensor integrated circuit (bukkit mod for servers), it flashes on and off during the night via repeater etc.
Dat Frog:
My mines of moria, I did remove all that stone by hand and yes it took a long time. Also I did it at the wrong height and got no diamonds.
Underwater dome, again by hand. Had help of two other people with this, used torches to create temporary air bubbles under the water to survive.
Map of our server:
http://dl.dropbox.com/u/40153836/map.png (giant thing to the west is the imperial city from oblivion, friend of mine is making, heruwar for ex-ravenous guys.)
Warning, big picture.