primecount is a command-line program and C/C++ library that counts the number of primes ≤ x (maximum 1031) using highly optimized implementations of the combinatorial prime counting algorithms.
Found via https://www.cliki.net/primecount and https://github.com/AaronChen0/primecount

F´ (or F Prime) is a software framework for the rapid development and deployment of embedded systems and spaceflight applications. Originally developed at NASA’s Jet Propulsion Laboratory, F´ is open-source software that has been successfully deployed for several space applications. It has been used for but is not limited to, CubeSats, SmallSats, instruments, and deployable.

AFF3CT is an Open-source software (MIT license) dedicated to the Forward Error Correction (FEC or channel coding) simulations. It is written in C++11 and it supports a large range of codes: from the well-spread Turbo codes to the new Polar codes including the Low-Density Parity-Check (LDPC) codes. A particular emphasis is given to the simulation throughput performance (hundreds of Mb/s on today's CPUs) and the portability of the code.
The comparison table on https://aff3ct.github.io/fec_libraries.html is interesting.
Note that the shaarli bookmarklet was working here.

RTags is a client/server application that indexes C/C++ code and keeps a persistent file-based database of references, declarations, definitions, symbolnames etc. There’s also limited support for ObjC/ObjC++. It allows you to find symbols by name (including nested class and namespace scope). Most importantly we give you proper follow-symbol and find-references support. We also have neat little things like rename-symbol, integration with clang’s “fixits” (http://clang.llvm.org/diagnostics.html). We also integrate with flymake using clang’s vastly superior errors and warnings. Since RTags constantly will reindex “dirty” files you get live updates of compiler errors and warnings. Since we already know how to compile your sources we have a way to quickly bring up the preprocessed output of the current source file in a buffer.

While existing taggers like gnu global, cscope, etags, ctags etc do a decent job for C they often fall a little bit short for C++. With its incredible lexical complexity, parsing C++ is an incredibly hard task and we make no bones about the fact that the only reason we are able to improve on the current tools is because of clang (http://clang.llvm.org/). RTags is named RTags in recognition of Roberto Raggi on whose C++ parser we intended to base this project but he assured us clang was the way to go. The name stuck though.

On-line references for the C and C++ programming languages and their standard libraries. Also available as archives for off-line viewing
http://en.cppreference.com/w/Cppreference:Archives

On-line reference to C++ and its standard libraries.

Mustajuuri (Black salsify, Scorzonera hispanica) is an audio signal processing application and toolkit. Written in C++. One of the targets of BlockCompiler.