That's funny I was just looking up Moxie and the art of Rule Breaking that Kate mentions the other day thinking the author looked really interesting. Elise Broach's books, already mentioned, are really cool too and she has a three-book adventure mystery series out too now.
I'll just throw in the Sammy Keyes mysteries since they haven't been mentioned here, I've only read the first but it's got a really nice first person voice and a not-too-big kid sized mystery.