In my opinion, there is no better way to expand your Atlassian skills than to prepare for one of the ACP exams. It is usually very motivating and gives you a purpose to learn also these less known features and functionalities.
If you are thinking about taking one of the available Atlassian Certified Professional exams, I have collected for you many resources which you may find useful when preparing for your ACP exam. I was looking for this kind of a summary myself, when I was preparing for my ACP exams. Since I could not find anything, I collected all these materials over a couple of weeks of research. You are lucky that you have found this article, because you will not have to do it again.
Links to study materials: https://www.atlassian.com/university/certification
Links to other materials:
ACP Exam Frequently Asked Questions - https://www.atlassian.com/university/faq/certification
Links to specific ACP Exams details - Jira:
Link to ACP Exam details - Confluence Administration
Link to ACP Exam details - Atlassian System Administration
Links to sample questions - Jira:
ACP 100 Sample Questions - http://atlassian.articulate-online.com/8679133671
ACP 300 Sample Questions http://atlassian.articulate-online.com/8679143319
ACP 400 Sample Questions - https://training.atlassian.com/static/Certification%20Sample%20Questions/ACP-400/story_html5.html
ACP 600 Sample Questions - https://training.atlassian.com/static/elearning/ACP-600%20Sample%20Questions%20Oct%201/story_html5.html
Some time ago I prepared a short video for those who want to know more about ACP exams. You can watch it on my YouTube channel here:
Another video that I recommend is this one, made by Ravi Sagar
Below you will find some links to articles on LinkedIn which I found useful:
1) Article by Vitor Fragoso on LinkedIn:
2) Article by Rachel Wright on LinkedIn:
I hope you will find my collection of materials interesting and useful. And also, I hope that I have convinced you to at least consider passing one of the ACP exams.