PBL (The Program Base Library) is a C library of array/linked list, hash/tree set, and key file functions that can be used in a C or C++ project. It is highly portable and compiles warning-free on Linux gcc, Windows Cygwin gcc, Mac OS X, and Windows Microsoft Visual C++ 2008 Express Edition.
Xbase (formerly known as xdb) is an XBase compatible C++ class library. It provides C and C++ programmers with a cross-platform class and function library for manipulating Xbase type datafiles and indexes. There is support for .DBF and .NDX files (including support for Dbase III and IV memo fields, dates, and record and file locking), and limited support is also supplied for .NTX (clipper index) files. There is also an HTML user interface class which works well with the Apache Web Server.
FramerD is a semi-structured object database integrated with a Scheme-based scripting language which supports multi-lingual programming (with pervasive Unicode), a stable module system for programming in the large, distributed applications (via an extensible RPC protocol), non-deterministic (PROLOG-like) evaluation for search and set operations, multi-threaded program execution, extensive tools for text and language analysis, built-in HTML/XML/MIME parsers, and intuitive (CGI- and FastCGI-based) Web scripting. The built-in object database robustly supports millions of objects and indexed access to those objects, both through disk files and networked servers.
OpenIsis provides a library and tools to access CDS/ISIS databases, which are mostly used for bibliographic data (ISO2709, Z39.50) but also well suited for other catalogues due to the very flexible indexing mechanism. Based on a C library, there are also bindings for Perl and Java (Tcl to come).