ashttpd was originally a testbed for a kernel async-sendfile patch. Today, it's just another Web server for serving static content rapidly. It differs from its competitors by storing its webroot in a database, which provides a 50% performance increase over using the operating system's VFS. It also allows precalculation of strong cache-validators (ETags).
cola is a C implementation of the COLA structure described in the paper "Cache Oblivious Streaming B-Trees" by Bender, Farach-Colton, et al. This algorithm is a drop-in replacement for B-Tree databases/indexes which performs faster on spinning disks than B-Trees do on SSD. It's asymptotically optimal in terms of data transfers to and from disk. For example, random key inserts are turned into linear sequential writes while maintaining the property which queries always require, at most, precisely log N blocks read sequentially (but not contiguously) from disk (or half that, on average).
Ozeki NG SMS Gateway sends and receives SMS, MMS, email, and voice messages via GSM modem connections or directly over the Internet (SMPP, UCP, CIMD2) to/from mobile phones. It is useful for SMS services like SMS notifications, two-way SMS systems, automated SMS services, SMS paging, SMS games, billing systems, email-to-SMS forwarding, premium rated SMS services, etc. It supports various APIs.
cmocka is a unit testing framework for C with mock objects. There are a variety of C unit testing frameworks available supporting different platforms and compilers. Some development requires a lot of different compilers and older versions, which makes it difficult to use unit testing frameworks. The idea of CMocka is that a test application only requires the standard C library and CMocka itself to minimize the conflicts with standard C library headers, especially on a lot of different platforms.
cqueues is a comprehensive event and networking library for Lua 5.2 and LuaJIT using modern Unix O(1) polling facilities. It includes libraries for buffered socket I/O, SSL/TLS sockets, DNS querying, signal handling, threading, file change notification, and X.509 key management. It natively supports Linux, *BSD, OS X, and Solaris systems without third-party dependencies, and is interoperable with any event loop that accepts plain descriptors, or is usable standalone.