Peter Regan of Oubliette Magazine has posted an article about Lego Dungeon Geomorphs. They may not be as affordable as geomorphs made with 3x5 index cards, but if you happen to have some spare LEGOs (or LEGO Heroica sets), they lend themselves well to dungeon-building.
I have been tempted for years to build some dungeons on the LEGO Minifigure scale for role-playing and war game use, but the cost of LEGOs and the inability to purchase specific Minifigures of my choice (instead of the mystery Minifigures) have precluded such activities until I win the lottery.
(Incidentally, what is the plural of LEGO? Is it LEGOs or LEGOes? Or LEGI? Or LEGAE?)