As C is the language that PostgreSQL itself is written in, it is very hard to draw a distinction on what is an extension function using a defined API and what is hacking PostgreSQL itself.
Some of the topics that we did not touch at all were:
contrib/hstore/for a full implementation of a new type.
pl/lolcodefor a language whose sole purpose is to demonstrate how a PotgreSQLs PL/* language should be written (see http://pgfoundry.org/projects/pllolcode/). You also may want to check out the source code for PL/Proxy for ...