In most cases in the US, unpaid internships are illegal, unless they meet certain requirements of the Department of Labor[1]. It mostly boils down to a question of whether or not the employer is getting useful work out of the intern or not (though the rules are different for non-profits).

I would guess that most EU countries have similar labor laws, but maybe someone with better knowledge can chime in.

1. http://www.dol.gov/whd/regs/compliance/whdfs71.htm