There are a lot of scams out there, and little newbies tend to have shakey legs and trusting hearts (awww). Here's a list in the UK that are legit:
If an agency isn't on my list, it doesn't mean it's not legit. It just means that I haven't dealt with them and can't recommend them. I've dealt with all the agencies listed in one way or another, or know someone represented by them.