if your searching especially for info about portable Scheme code, following places might help:

• slib has an .init file for all supported Schemes with some compatibility code; less common procedures, but rather meta data; could still be equally interesting for the implementation-metadata repo
• the r7rs-benchmarks repo has some src/xxx-prelude.scm files, this time more like Scheme-specific procedures for a common layer
• Oleg Kiselyov has some common prelude procedures for various Schemes here; not exactly up-to-date (2004), but hey: it's from Oleg
• I also once bookmarked the prelude procedures in this blog post, but I assume that you could find many of the procedures there in SRFI-1.
• If you like if-let, when-let, push, ... and some more CLisms, they are here.
• If you're searching more generally for Scheme best practices, I don't have any links; I like to read through the SRFI implementations, because that's often so beautiful code.