It's a common knowledge that since the IT industry develops very quickly, most of the skills get outdated fairly rapidly: jQuery, .NET, Ruby on Rails, etc. They change all the time so that it is difficult to keep track of them and especially learn them. Once I learn Ruby on Rails 3.2, Ruby on Rails 4 has appeared; the same thing for other frameworks and libraries.
What skills (or rather, technologies) get outdated slower and, at the same time, pretty useful in terms of being able to find a job? I figure these are knowing networks and algorithms. What else?