Your interviewer wants to know how familiar you are with the industry's applications. They also want to know if you intentionally stay up-to-date on new programs, tools, and product developments.
You can list the most popular tools you currently work with or have worked with in the past, and mention whether you have worked on proprietary systems. Mention any skills with less common programs that are in demand.
Avoid mentioning programs that you haven't worked with for 3-5+ years, or any system you aren't comfortable enough to have a user-level technical discussion about. Also, don't mention proprietary systems by name unless they were built on a well-known framework where your knowledge could transfer.
"I've worked on CRMs like Salesforce and SAP, and other proprietary programs. I'm also very familiar with Wordpress, and content automation services like Buffer and Social Pilot. I take time out to review product release emails I receive to stay aware of new features in the programs I use regularly. I read new tool reviews on blog posts that I come across while researching new solutions for my clients. I also watch at least one video per week on my free time about marketing strategy; this is another way to hear about new tools and tricks this."
"Currently, I'm working in a number of customer relationship databases, design tools for content creation, content management system, marketing automation and analytics tools. I follow industry leader newsletters, listen to podcasts, and occasionally visit tradeshows to stay on top of upcoming technologies in my field."