Talk to them, briefly. Don't burn bridges, but also don't waste your time (and theirs). When you identify a good one, hang on to their contact info.

How do you identify a good one? Well, I once got called by a headhunter, whom somebody gave my name to. One question he asked is where I was currently working. I told him. He said that he couldn't recruit me, because his company placed people where I was working. A recruiter with ethics? I put his name in my permanent file. If I decide I'm looking for work, he's getting a call.