STL and Boost are musts.
SQLite provides a completely embedded, full-featured relational database in a few 100k that you can include right into your project. It’s also a highly marketable skill because of its high presence (it’s included in Mozilla Firefox as well as Android and iOS).
If you’re interested in creating user interfaces, look into ncurses — it’s the library that was used to create many terminal user interfaces and can be very useful for creating games and shell utilities. Qt is a good GUI framework for C++.
If you’re interested in graphics or creating games, consider SDL or OpenGL (or DirectX if you don’t mind only working on Windows).
Of course, there are thousands of interesting libraries. It really depends on what you’re interested in.