For me, it would have to be Tie-a-Tie. I wear ties so infrequently, that I can never remember how to tie them right. However, there are times where I obviously needed one (my wedding, job interviews, etc). This website has been a minor blessing.
Sigh, I need to install Flash to enjoy it. Nope. My memories need to be updated to HTML5. Might look into that.
Not the original, but at least you get a hint of the humor of the internet in the early '00s.
Ah! I remember that from when I was a kid. People used to post stuff like this in public forums. Hey, that's our culture right there. As much as we might despise it as low-brow, it's part of our history now. Why does it feel so good to be in touch with one's history?
This is pretty old and stupid, but I still go there once in awhile - "fly guy".
The Common Lisp Hyperspec. It's the ANSI Common Lisp standard, htmlized by hand by one of its editors. The Online Encyclopedia of Integer Sequences is really handy if you have a few terms of a sequence from some problem, don't recognize it, and want to see what's known about it. Discogs.com knows all.
This is esoteric for the average person, but www.imslp.org is a lifeline for classical musicians looking for music, or musicologists looking for facsimiles of music manuscripts. for the simple, probably http://oblicard.com/ which is a digital version of the Oblique Strategies