900 numbers are used to charge money to the caller that is sent to the recipient of the call. It's basically a way to pay for a phone service without having to use a credit card. It could be used for out-of-warranty technical support, but, as John says, it's mostly used for phone sex and psychic predictions.

The only toll-free ones are 800, 888, 877, and 866, though I wouldn't be surprised to see more in the future. As I've explained, the caller pays nothing for the phone call and the recipient pays for it. I've never paid a toll-free bill, but I believe you generally have to pay a monthly fee for the number to be active, plus a per-minute charge that is probably about the same amount the callers would have paid for the same call if it were a normal long-distance call. If you're looking at giving these numbers to your subscribers, that's probably a bad idea. Toll-free numbers are pretty much exclusively used for businesses. While there's no rule that says that has to be the case, an individual with a toll-free personal number would be strange.

As far as I know, there are no other special area codes. Also, people wanting to get a phone number from you are going to want to get a number with a local area code.