I recently found a web based tool to help me edit my minecraft ssp worlds! It allows you to instantly install mods from hundreds of the most popular mods, edit in-game inventory, change your game mode on the fly, and much, much more. I've been working on a castle for a long time, and I used nogolo to let me paste in .schematic files, so I finished the castle in not time :D

Nogolo's really easy to use and it's a must have for any hardcore SSP'er Try it out now at nogolo.tk