In addition to MCEdit, which reads and writes Minecraft worlds offline, there is WorldEdit, which can be used in-game (whether via a client-side mod such as Sin

In addition to MCEdit, which reads and writes Minecraft worlds offline, there is WorldEdit, which can be used in-game (whether via a client-side mod such as Single Player Commands or via the official server-side plugin). If I recall correctly, with CraftScript installed one can also automate block placement. With the right config you can make this all work in SSP or SMP - WorldEdit will use blocks from one's inventory instead of creating them ex nihilo.