I've never signed up for these kinds of services myself, but I've been in the same position you are now, and so I have researched them. Again, don't take my word as a firsthand account, but what I have been able to gather is that, while some of these sites are a scam, the premise itself is not necessarily a scam. However, you should not expect to make livable money from this. At best, it'll earn you some petty cash on the side. Any website that offers you ridiculous sums of money in a short period is probably a scam. Any website that wants you to pay to sign up is almost definitely a scam. If you do sign up, and hand over your credit card number, keep a watchful eye on your account for fraud.

Look at the websites. Look for clues that they're real or fake. If they have a phone number and a physical address, that's a good thing. If they have only a couple pages on their whole site, and a shoddy-looking design, that's a bad thing. Common sense stuff.

Look for a job AT school; most educational institutions have stuff for students to do. Granted, none of them might suit you, but they like having students doing the administrative/secretarial work, because it lowers their staff costs (students cost less) and increases enrollment (more students can afford to attend).

And I echo Josh on whether or not it's worth it. A physical address, and the question of whether you pay up front, are paramount factors in asking whether or not it's a scam. Since when, after all, have you paid to take a job at a company? Any "signing costs" should be mitigated by profit brought by the employee.