For years and years now, David Welbourn has hosted a set of the best walkthroughs I’ve seen in IF-land. They’re generally well laid out and divided into handy sections so that you can instantly find the part of the game you need to look at. Sometimes, he adds commentary explaining the concept of a difficult puzzle, or putting together the pieces of a plot; in several, he distinguishes between commands that you need to follow in order to understand the game completely and a minimal walkthrough that will get you through to the end quickly. Quite a few of the walkthroughs also feature maps.
For an example of the level of detail he lavishes on a complex piece, check out his handling of Andrew Plotkin’s The Dreamhold.
The other awesome thing about this walkthrough set is its diversity. A lot of favorites and well-known games appear, but Welbourn’s extensive catalog also includes a healthy array of Speed-IF, minicomp entrants, and games that otherwise might be very hard to figure out on your own.
I think he’s been focusing on some other areas more recently, as the walkthrough set doesn’t cover very recent years. But it remains a significant resource, especially for older games where it might be hard to find anyone on the forums who remembers a given game well enough to offer hints. And it’s also a great place to look for people who are interested in making friendlier, more followable walkthrough formats.
(Edited to add: at first posting I had linked accidentally to an older version of this site; the current link is better. Sorry about that!)