NCBI provides several public software libraries that developers can use to manipulate NCBI data.
NCBI C++ Toolkit
The NCBI C++ Toolkit is a cross-platform application framework consisting of utilities and classes for working with biological data. The Toolkit contains source code for NCBI software, such as BLAST and Cn3D, along with many tools for manipulating ASN.1 data. Extensive documentation is available, along with a source browser and full ASN.1 specifications.
The SRA toolkit is a set of compiled binaries and corresponding source code for tools that download, manipulate and validate next-generation sequencing data stored in the NCBI SRA archive. The binaries are available for Windows, Mac OS X and LINUX platforms.
NCBI GitHub Repository
NCBI maintains a software repository on GitHub.com that contains a growing collection of code from various NCBI products such as PubReader, SRA and OSIRIS.