I have recently found wikidPad and it seems like a very useful tool. It is an offline wiki, the pages are stored either as individual text files (allows searching by external tools) or in a sqlite database (less files), and it has some interesting features I am still exploring.

I believe that a good use of such "external memory" can make people much more efficient. And this seems like a universal tool; you could have notes, contacts, to-do lists, internet bookmarks, and other things in one system (which allows easy hyperlinks between them).

First time I realized an importance of making notes was one year after finishing university. I found at home a paper with a list of exam questions... and it was a shock to realize that at half of them I didn't even understand what the question was. But those were all things I spent years learning, and now the knowledge is... gone. Similarly with programming; when I don't use some programming language for a few years, when I try again, I have to learn many things again. Now I keep written notes and example pieces of code.