[krylon]

When I started in my current job, I was effectively the single administrator (servers, network(s), clients) at a small-ish company (~70 employees using PCs), and there was a landscape that already was the result of 10+ years of growth without any central planning.

TL;DR - I was thrown into a relatively complex environment with little help other than asking the two guys who had done this before me, both of which were rather busy (they had done the administration a side-business of sorts). (I still wonder how they managed to keep the IT infrastructure running, but in my first few months, many of our users told me how happy they were that I responded to their calls for help right away rather than "sometime next week".)

I used emacs' org-mode. I kept a file where I wrote down every little bit of information I could gather. After 12, maybe 15, months, I had memorized everything I needed to know and stopped taking notes. So my example might not fit your question all that well. But for the time I was getting to know this company's infrastructure, its servers, networks, and people, that .org file was a lifesaver.