Learn what you want, i can't say what do you want to learn, i can only say the things i like:
1) I love how Qt works, signals and slots mechanism is very practical and easy to read. (C++) (KDE is made from it)
2) .NET is the best framework i know, very stable, very good architecture, and intelligence sense of Visual Studio is just marvelous. (I don't like gui system)
3) Core Image is pretty neat (http://developer.apple.com/library/mac/ ... rence.html
). Objc one of the best things to work with 2D graphics and images
4) XNA the best for 3D, C#
5) Command line, bash scripting, etc. is very powerful in good hands.
6) Linux kernel modules, hard coding for understanding a system.