Mary and Matt have created a wonderful book featuring Thank You bags called You're Welcome. It seems like one of those ideas that has never ending possibilities and isn't terribly hard to do (don't take that as downplaying the terrificness of this), which is the perfect combination.
I put this in the category of I-could-have-done-that-but-didn't-so-they-are-better-than-me. That category is starting to get a bit crowded. Click here for more of their project.