Linux does not have anything akin to ActiveX, if by that you mean code that you can put on a website (which is inherantly insecure).
There are several libraries for GUI programming in Linux, each with their own strengths and weaknesses. Qt is good for basic widgets, database connectivity and OpenGL. The various KDE libraries extend Qt with support for much more, including internationalsation, autocompletion, and high-level things like spellcheckers. The Gnome libraries are slightly harder to work with from a C++ viewpoint as they are written in C, but they tend to work much faster, especially outside of KDE. Motif is lightning fast, and nt provides GUI-style widgets in a text console.
For socket/networking programming, try looking at the POSIX socket library (/usr/include/socket.h)
Good place to look for useful widgets/libraries are www.sourceforge.net