If you couldn't tell by my previous post, I absolutely love in-game editors. If the game concept is fun, ala Boom Blox, then creating your own levels extends the life of a game immensely. Kotaku just announced that Scibblenauts will also have a level editor and a way for you to share your level creations with other DS owning friends.
To top it off, as if this news isn't already awesome enough, you can actually customize the relationship between items in the game. So if you were to build a level with a large body of water you could make all fish afraid of water. And for some odd reason, you could make a tiger your only option to get across the ocean. Not sure why you'd want to do that, maybe it was a bad example, but the limitations are endless.
Trust me when I tell you I will be playing this game at E3 and giving you some hands-on impressions. It might be the only game I write anything about specifically.