This may include code which is not just libraries, but that is pending better organization.
anyhow seems fairly heavily used throughout Rust projects. It was one of the first libraries I came across while starting to work with Rust in earnest (through working on gstreamer). The code is nice and clean with well-documented key files, and the project overall seems like a nice demonstration of how to implement a compatibility layer in Rust. For narrow uses it seems as though making use of the standard Error trait may be sufficient, but this certainly seems like a valuable tool for building maintainable, portable software which is likely a core aspect of the Rust model when compared to providing or expecting a broad standard library (which is something I'd want to find or write a description of at some point).
exec-path-from-shell is a fairly widely used module to make sure that values from interactive shells are available to Emacs on OS X and elsewhere. The logic is a fairly straightforward invocation and parsing of an interactive shell. The code itself is small with most of it being in support of adapting around use so this can act as a readily usable module rather than more of a code library (which could be reduced to a few small functions).
bit-struct is a library I quickly skimmed through while looking at options for C-style bitfields in Scheme. It provides a range of macros to provide this type of functionality but I ultimately landed on the opinion that composed logic for serialization/deserialization would be far simpler.
yaml-rust provides YAML parsing and emitting in a form that at least seems worth repurposing. The most up-to-date YAML logic for Rust seems potentially elusive, or this project is just old as it has remained good enough. The code is a fairly specialized state machine type flow with a split between lexing/scanning and parsing. I'll be looking to build on this as part of thoughts around application programming, but may also look at generalizing some of the behavior a bit (and also looking for a more grammar-y alternative.
gash is a fun project to implement a POSIX shell in Guile. There's plenty of ideas to borrow from in the code, though my evolving opinions steered me away from this project for the time being.