From the earliest version I saw, Lucian’s story file has included:
Include Version 7.1.1 of Hybrid Choices by AW Freyr.
Include Basic Screen Effects by Emily Short.
As I’ve alluded to above, extensions are a means of bundling code and the include assertion has an effect similar to inserting that extension’s contents in its place. While they are a useful way to organize one’s own source, a lot of the motivation behind their design was to facilitate sharing, so you can use someone else’s code.
Extensions have been an Inform 7 feature from the start. One of the inclusions above, Basic Screen Effects adds several important UI features; its use is demonstrated by several examples in the docs. For years, it and a bunch of Emily Short’s extensions (e.g., Locksmith, Basic Help Menu, and Punctuation Removal) have been bundled with Inform, along with Rideable Vehicles and others by Graham Nelson, and Eric Eve’s Epistemology.
A huge number of other extensions have been published over the years, many of which are amazing. It’s also the case that many have depended on officially unsupported low-level details in the compiler and/or the Inform 6 layer. The documentation explicitly warns against relying on these because they were subject to change without warning.
Turns out it really meant it. Many extensions that had been popular in one release didn’t work in the next. Some were updated, but there was no reliable way to look up whether a given version really worked in the current Inform release. Your best shot of finding out had been to search here hoping for recent reports. A couple of times, I’ve heard people dismiss even trying a third-party extension because of the likelihood it wouldn’t work.
And that’s a shame because for some years now, we have had extension collections organized by corresponding Inform release. Here are lists with capsule descriptions for easy browsing:
AW Freyr’s Hybrid Choices is a popular choice for implementing choice-based navigation in Inform. Thus far, our challenger doesn’t seem to be using it but it remains an intriguing sign of intent or aspiration.
For much more, see Where to find Inform 7 Extensions.