Even though there are many legitimate websites, it doesn't change the fact that there are still scams out there. I highly recommend doing your research before committing to a website. I also recommend NEVER paying to join a website. These might not 100% be scams but I would be VERY cautious about them.
Real Writing Jobs - I recently received an email from them, I don't remember applying to them so I did a quick search. I found out a lot of negative information. They are constantly spamming people.
Freelancer - The actual website itself might not be a scam. However, I've read some very nasty reviews and comments about them. I decided to stay away from them completely. You can research the internet for tips about avoiding scams on Freelancer if you wish, but it's too risky for me.
Daytipper - I have read 2 yahoo articles saying that people were never paid.