Seeing ZZ Top play 'La Grange' for free is the highlight of both my tech and musicgoing career.
Bout you guys?
Skipping school/work for a week with my then-girlfriend in January of 2001 to join the mass street protest against corrupt Philippine president Joseph Estrada (http://en.wikipedia.org/wiki/EDSA_Dos), where we were serenaded by renowned folk-singer/human-rights activist Noel Cabangon (http://youtube.com/watch?v=eAuMfsPnxNg) (among others), all the while staring down the very real prospect of being fired on by the military.
A distant second would be watching Filipino punk/folk/reggae group The Jerks (http://youtube.com/watch?v=z9Hi9tJ79Uo) during any number of their various free protest concerts (mostly to protest globalisation and the militarization of the state).
I've really attended way too many free concerts (and my hearing's all the worse for it) since I turned 14.
Oh man, the Sinaloa festival for culture and arts. We got to see opera, tangos, percussionists... every other night of the week there was a free concert. The two highlights was getting to meet Ely Guerra who greeted me with a hug and kiss on the cheek (everyone I know has a crush on her) and getting to meet and hang out with Lila Downs after her show.