This turns out to be more important that I thought would be necessary when I was a beginning programmer. Often code that I wrote for myself would go uncommented because, after all, I was the only guy who would ever read it. This worked out ok as long as I came back to the code within a few weeks after I wrote it, but that's often not the case. I've mentioned dodecahellspawn, and what I'm going through reworking that, but I have even older code that I still work with regularly.
The oldest application that I've written, and still use regularly, is Notepadplus. As you may be able to tell from the name, it's a basic text editor based off of Windows' Notepad application. The main point was that it used a rich edit control instead to bypass the 64K text limit that Windows 95's Notepad had. This was very necessary for use as a code editor. Specifically I used Notepadplus to complete many of my machine problems and do some web page editing. That was back in 1997 after I had changed majors to comp sci in undergrad. The last modification was done a little more than a week ago.
And now I'm actively laying down code that I fully intend to be using five or more years from now. I think this time I'll comment the damn code.