Luckily this happened some years ago and I'm in a better position now (not perfect, but certainly better than job hunting).
I was looking for tutoring jobs in the Bay Area — being a freelancer tutor for certain subjects can really rack up the funds if you get the right clients — when a company emailed me about doing a “trial run” at one of their offices. Long story short, I'm there for roughly an hour supervising a group of kids doing math homework without anyone looking in. It became pretty obvious they were cycling through 'candidates' for free labor an hour at a time.
Since then, I've become completely unwilling to send in any work a company requests on the spot as part of an application, e.g. writing 1-2 pages on a certain subject, knowing there's a non-zero chance they're just a slimy firm exploiting job seekers.