I have a restaurant client who asked me a few years ago if I would come in and 'over lunch' teach his junior office assistant how to update and maintain their site, as a favour, to save them money.
I responded "certainly, in return I'll come in the following day and 'over lunch' you can teach me everything there is to know about being a chef'. Funnily enough he never asked again and years later is still paying me to update and maintain his site.
Same as some of the other posters above, I do the occassional freebie but only for non profit groups that I'm involved with. I find it insulting that people in business would expect my time for free and will quite happily say so.
p.s. I'm a middle aged hippy chick, proud of it, productive and hard working and sometimes I can even look respectable