I won't ever tell you I've "mastered" something because I learn new stuff all the time, as anyone who's paying attention should, but I do think I'm pretty good at a few things.
Some of my favorite things to learn about:
- The typical "Alt.Net" Toolbox (DI, AOP, ORM etc).
- Agile Practices & Techniques (TDD, CI, Unit Testing, Mocking etc)
- Domain Driven Design & Design Patterns in general
- Developer tools such as Build Servers, Code Coverage, Automated Deployment, Source Control, and Work Tracking. Lots of experience with TFS, but I very much prefer Hg, Cruisecontrol etc.
- Objective C and Mac/iPhone/iPad development