a free user interface builder for GTK+ and GNOME interfaces designed are saved as XML, and by using the libglade library these can be loaded by applications dynamically as needed. (Glade can also generate C code)
a source code tag system that works the same way across diverse environments. You can locate a specified object in the source files and move there easily. It is useful for hacking a large project containing many subdirectories, many #ifdef and many main()
Guy Steele's keynote at the 1998 ACM OOPSLA conference on "Growing a Language" (mostly about JAVA) discusses the importance of and issues associated with designing a programming language that can be grown by its users.
an interface for array-oriented data access and a library that provides an implementation of the interface. The netCDF library also defines a machine-independent format for representing scientific data.
a library of routines for managing a database. The database is a simple data file containing records, each is a pair of a key and a value. Every key and value is serial bytes with variable length.