BurntSushi's repositories are generally very well-written examples of rust code, and cover at least one base - algorithms. In particular, regex, and its dependencies, like regex-syntax and aho-corasick are good examples of very fast, yet well written algorithm crates. ripgrep is a high quality (and also very fast) command line tool which uses crates like these.
On the RDBMs side, this isn't SQL, but I've found sled to be quite a nice repository to use, and also to read. It's a full-Rust embeddable nosql database, with, as I understand it, novel data structures, and an implementation that could eventually compete evenly with the likes of RocksDB.
Cross platform GUIs, there are lots of incomplete crates, but I don't think I'd rate any of them as particularly high quality. Or at least, I've used two or three of them, and been slightly horrified at their internals. They're not that bad, just definitely new, and many refactors away from a clean codebase. I think, since this is a huge space and there's a lot to implement before projects get "done", features might be prioritized over cleanliness?
With that said, I've heard good things about druid and orbtk. And since I've used neither of them, I can't claim I know they're bad .
Finally, on vector graphics, I think lyon is the best/furthest crate out there. I don't know enough about it to rate its codebase's cleanliness, but I know there's been a ton of work put into it and the API is quite nice.