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 exactly what it sounds like, a checklist of all the skills you will need to master before taking the certification exam.
This blogger shares her Google Certification process.
I haven't actually taken these practice exams yet, so I don't know how helpful they will truly be. They are free, so I plan to try them out in the near future.
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.