As a Fulbright ETA at Jungwon High School (a coeducational public school in Bucheon, South Korea), I taught English as a Foreign Language (EFL) from 2003 to 2004. Most of the lesson plans and materials I used (to teach conversational English to Korean high schoolers) can be viewed here.
Before that, I was a volunteer instructor at the Head Start Community Center in Jamaica Plain, Massachusetts, where I taught English as a Second Language (ESL) classes for recent immigrants from 1999 to 2003.