It's a Javascript (obviously, since it needs Javascript to run ). It's not written by me, I just modified it some to suit my own mapping needs. It just places lots of small 32x32px graphics on a grid (the size of which, and even the size of the squares can be modified, but 32x32 seems to work well enough).
BTW, if you havn't noticed, if you leave the mouse hovering over a square, you will get a tooltip with some information about that square (works best in IE, since Firefox has some issues with showing long tooltips; there's an extension out for that, tho)
Exp.! Thanks. I like the format, it is pretty cool and I may try it in a future game. It is an interesting contrast to a friend of mine who likes to keep everything, including all discussion and characters in one thread.