This resource is my favorite. I found it on ShakeUpLearning.com. It is a table with the different Google apps or concepts and resources to click on for each to familiarize yourself with them. There is a section where you can also mark your comfort level, so you know which apps you should review or spend more time on before taking the test.
This link is probably the most trusted and reliable resource as it is straight from the source. Google offers trainings to prepare you for the exam. There are thirteen units with multiple lessons including videos and other resources to help you be successful when taking the exam.