Rcpp looks very interesting, letting you use R from inside a C++ program.

GNP.h. A simple wrapper to drive Gnuplot with popen.

Gnuplot-iostream interface. Drive gnuplot with an iostream pipe.

ROOT. A data anlysis framework largely written in C++. Used by CERN.

Silicon 0.1. Write HTTP APIs in C++.

GNU Libmicrohttpd. GNU libmicrohttpd is a small C library that is supposed to make it easy to run an HTTP server as part of another application (their words, not mine).

socat - Multipurpose relay. socat is a relay for bidirectional data transfer between two independent data channels.

GnuTLS.

LWAN. Lightweight asynchronous multithreaded event-based web server. Written in C.

Sub-processing with modern C++

Google C++ Style Guide. This has been evolving over the years, and is becoming something I can agree with.

Jinja2. Templating library for Python. Use it as a model for a C++ template library?

cppit/jucipp. juCi++: a lightweight, cross-platform C++-IDE supporting C++11, C++14, and experimental C++17 features. Lots of dependencies.

cppformat. Small, safe and fast formatting library for C++. Github repo.

P0067R1: Elementary string conversions, revision 1. No implementation yet?

A printf-like Interface for the Streams Library

RECURSIVELY WALKING A DIRECTORY WITH C++XY (FILESYSTEM + YIELD) [SIMILAR TO OS.WALK IN PYTHON]

Guidelines Support Library Review: span<T>.

Error Context - a stack trace for data.

C++14 standard. Approximately; from late 2013.

C11 standard. Late committee draft.

Phases of translation. Slightly more readable than the standard. The preprocessor should be identical between C and C++.