Why don't more people do this? Im 24 and am pissed I just found out about this now!!
That is not traveling for free. You work for a family and take care of their kids and some other stuff. It's not really as fun as you imagine it to. Besides it's only limited to western countries more or less. So you can't exactly go to South America.
Teaching English is another great way to travel. It's not 'free' because you are working, but total independence, awesome co-workers from around the world, a pretty decent salary and perks like an apartment, visas, insurance etc. Its harder to hop around all the time, but every 6 months is definitely doable. In some countries you only need to work about 15-20 hours a week to pull that off and have savings!
It's not traveling for free. Au pairs work as child care givers for a family, and cannot come and go as they please.
Sounds like fun. Look into it and also check couchsurfing.com.
