baysupersitters.com - you pay a one year membership and they give you a list of resumes of people they've already checked out and screened. You only need to pay the first year and can keep the list of resumes. If you pay the next year, you just get access to new resumes. If you don't, you can continue to use the ones you already have.
thecoregroup.com - we use this for emergency babysitting where we they have an army of sitters that you can call last minute for availability. This is great if you have to go somewhere the next day and your sitter bails or didn't have the lead time to set it up. They might also be able to get you someone same day. Their sitters are checked out and fingerprint. We've had good luck the few times we've used them.