Swarm is high-perfomance library for web crawling.
Depends on libcurl, uriparser, libxml2.
libcurl built with --enable-ares is highly needed for better perfomance. libcurl must be built with SSL support for accessing https sites.
More information is available at Reverbrain
TheVoid is asynchronious event-driven C++ library for building high-perfomance web applications.
Depends on boost::asio and swarm.
More information is available at Reverbrain and ReadTheDocs.