[seanb]

Cling, from CERN, provides a REPL: http://root.cern.ch/drupal/content/cling

There are also clang-based auto-complete and syntax-checking plugins for sublime, emacs, and presumably others.

Another emacs package I've been meaning to try is "disaster", which shows you the assembly that the code under your cursor compiles to: https://github.com/jart/disaster

What c++ is sorely lacking, in my opinion, is a (library) package management system.