Science! Engineering! Computers! These are the things that I love, and these are the things that I teach.
I'm a software engineer, among many other things, and I both love and miss teaching people what I know - and I know quite a bit about a great many topics; but chiefly, programming, mathematics, and physics (as I went to school for electrical engineering, but landed in software.)
In addition to the topics on the side, I can teach Perl, Assembly, and Embedded Programming.
The best teachers are those who are passionate about their topics, and who can inspire - then sate - curiosity in their students.