Portable but advanced things:
Time types: SRFI 19 and/or TimeAdvancedCowan with TimePeriodsCowan
Binary I/O: BinaryIoCowan
Character conversion: CharacterConversionCowan
Parallel promises: ParallelPromisesCowan
Pathname objects: PathnamesPython
URI objects: UrisGauche
Unicode character database: UcdCowan, AdvancedUcdCowan
Environment: MiscEnvironmentSchudy
Trees: TreesCowan