For quite some time now, I've kept my employers confidential due to concern over being targeted by foreign entities in relation to my work I've performed for various Federal agencies. It's been an interesting run, providing my expertise to Defense Information Systems Agency, Department of Veterans Affairs, the Census Bureau, and various private companies over the years. I've worked with state-of-the-art computer systems distributed across the country, as well as worked with computer systems which are actually older than me but still up and running (and I just recently turned 40).
Three books, and thousands of tweets later, and I still feel like I've only scratched the surface of the real depth, and scope, of the cyber security realm.