SPGSQL is the finest pure Scheme PostgreSQL driver you'll find for PLT Scheme
SRFIs are ported to PLT Scheme by members of Schematics, and distributed as part of the core PLT Scheme system.
Instaweb is arguably the fastest way to get the PLT Scheme web-server up and running.
SchemeUnit is the premier unit testing framework for PLT Scheme.
Benchmark is a SchemeUnit extension for testing code performance.
Simple Improvement is a process improvement extension for SchemeUnit.
XML-RPC is the best XML-RPC library you'll find for PLT Scheme.