Gow (GNU On Windows) is a lightweight alternative to Cygwin. It uses a convenient Windows installer that installs about 130 extremely useful open source Unix applications compiled as native Win32 binaries. It is designed to be as small as possible, about 10 MB, as opposed to Cygwin which can run well over 100 MB depending upon options.
The Geospatial Data Abstraction Library (GDAL) is a unifying C/C++ API for accessing raster geospatial data, and currently includes formats like GeoTIFF, Erdas Imagine, Arc/Info Binary, CEOS, DTED, GXF, and SDTS. It is intended to provide efficient access, suitable for use in viewer applications, and also attempts to preserve coordinate systems and metadata. Python, C, and C++ interfaces are available.
The libmba package is a collection of mostly independent C modules potentially useful to any project. There are the usual ADTs including a linkedlist, hashmap, pool, stack, and varray, a flexible memory allocator, CSV parser, path canonicalization routine, I18N text abstraction, configuration file module, portable semaphores, condition variables, and more. The code is designed so that individual modules can be integrated into existing codebases rather than requiring the user to commit to the entire library. The code has no typedefs, few comments, and extensive man pages and HTML documentation.
Haystack is a powerful tool designed to enable each and every individual manage all of her information in the way that makes the most sense. By removing the arbitrary barriers created by applications that only handle certain information "types", and recording only a fixed set of relationships defined by the developer, users can define whichever arrangements of, connections between, and views of information they find most effective. Such personalization of information management will dramatically improve your ability to find what you need when you need it.
dns.c is a recursive, re-entrant, non-blocking DNS resolver in a single .c file. It features a record-agnostic API, restartable record iterators, and smart queries which recursively follow out-of-bailiwick NS, MX, and SRV host references. It works just as well as a stub resolver, too. It also includes spf.c, a non-blocking SPF resolver with no threading, no forking, and no run-time dependencies.
Shapefile C Library is a C library for reading and writing ESRI Shapefiles and their related DBF files. All geometry types are supported, with robust DBF support. Shapelib is widely used for commercial and free projects. Shapelib includes command line utilities for dumping, subsetting, clipping, shifting, scaling, and reprojecting shapefiles.