One of the functions that I personally find most useful - I use it in almost every program that reads free form input - is getline.c This function reads in lines of unbounded length and also compiles to a version that does comment processing.
Another useful function is getarg.c which is a getopt replacement. The function xstrtok.c is a reentrant version of strtok(3).
Here is a little program called num.c Which reads a number in and gives it back in binary, octal, hex and decimal forms. The input can be in decimal, octal (leading 0) or hex (leading 0x.)
Here is a program similar to date called pdate which has a few extra features that are useful.
The file ascii.h is a header that defines symbolic names for all the control codes and, for MS-DOS systems, all of the alternate keys such as arrows and function keys. The comments explain how to use these values.
I have created a preprocessor called generate which is available as a shar file or as a compressed tar file. Here is the readme file.
I have modified PyGres95 and make it available as PyGreSQL.
I have a chapter in the book "Software Solutions in C" by Dale Schumacher, Academic Press, ISBN: 0-12-632360-7. It includes a discussion of and the source for cash.c. This code is the base for the MONEY type in PostgreSQL RDBMS.
Watch this space for more source as I get around to cleaning it up and putting it here.