Red Kobold is running a Kickstarter campaign for their Blue Dungeon Tiles which can be written on with both wet and dry-erase markers.
From the campaign:
Blue Dungeon Tiles are map tiles for roleplaying games and miniature wargames. To provide maximum versatility, the tiles are double sided with common corridor configurations on the front face and a grid on the back face. The tiles can be assembled into rooms and corridors in moments. The grid and designs are marked in increments of 1″. The tiles of the BASIC set are 4″x4″.