5. C/C++ or Assembly. I struggled not to leave this as Assembly — or even C and omitting the mongrel hybrid cousin (Linus said it best in his NSFW rant). You needn't be a master of the standard lib or the STL or any such thing, but you should know how a computer actually works. You should understand how memory operate close to the metal and have some idea of how compilers function. I may not have used MASM in years, but I've continually applied the knowledge I gained from learning it. As technology grows more high-level and abstract, understanding them at a low level actually becomes more useful when developing or debugging highly scalable, high-performance systems. If you're working as a Ruby developer, no one is going to demand you know this stuff, but they may ask you to fix problems that you'll understand far better if you have this expertise. People who can think this way will prove themselves invaluable time and time again.
6. Git. Look, if you don't know Git and haven't set up a GitHub account or two, you're late to the effective and well-known developer party. You really should've learned it last year. Of course, if you're still using ClearCase in your current position, you should either be getting serious hazard pay or you should quit and get a job where you don't have to use ClearCase.
Lots of other skills are in demand, but these six are bringing the buzz. Acquiring these skills will help you become not only one of the cool kids on the block, but also one of the paid kids.
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