FOAF documentation for developers...
For general RDF and Semantic Web developer documentation, the W3C RDF homepage, and ESW Wiki and Semantic Web homepage are good starting points.
Ian Davis has written a very useful document on Parsing FOAF with PHP, which should be of general interest even for those not coding in PHP.
Similarly, the HP Labs Semantic Web group have a Java toolkit called Jena, for working with RDF. The Jena tutorial provides a good introduction to RDF theory and practice.
foaflib is a Python library for parsing and creating FOAF files. It can also generate activity streams and scutter linked profiles.
Also, the W3C RDF Validator usually comes in handy.
