Well, I'm using XP so it might be a different location.
C:\Documents and Settings\DerpName\Application Data\.minecraft
There will be a unique file for each server, the name will include the IP.
Anyway, here is a example of this file.
!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;
First number is the item ID located from
http://copy.bplaced.net/mc/blocks/ and
http://copy.bplaced.net/mc/items/Second is the quantity.
Third is the damage to the weapon.
46,64,0;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;!;
This means you will have 64 Tnt with 100% durability on spawn.