I'm at a cross-roads in my professional life. I've dabbled in various languages/technologies (LAMP, iPhone apps, c++) and programming activities (contests, writing C# code as a "tool developer" for a living, work on some simple web apps etc). I can learn pretty much anything but I'm far from mastering any of these domains.
What technology would you master if you had practically unlimited time and resources?
Why that particular technology? Job security? Benefit to open-source community? Interesting?