Let me start by saying in any of these types of positions you are not gauranteed the hours. The one exception is that some agency's have contracts where you will be paid even in the event of cancellation by the facility, however, most hospitals are not too keen on this type of contract. In agency work, there are contracts for say 13 weeks, 8 weeks ect., these types of contracts generally involve weekends and holidays. They can also call you on a as needed basis to fill in at facilites that they have orientated you to, these positions allow you greater control over your schedule. Depending on the agency, your contract will state that you can not apply to work directly for the facility for 1-2 years. (a big downside and something to think about) Pay range is all over the board, the agency I worked for paid $36 per hour, another I interveiwed for was only $30. Optimal is one of the bigger agency's. Contigent and per diem are pretty much the same thing. I was lucky to work at a hospital for 5 years as a contigent RN with no weekend or holiday commitment, sadly that has changed and why I went to the agency. These positions generally involve a hour min., one weekend per month, holidays, and at some places "off holidays". For instance, Henry Ford's policy is you must work 120 hours every quarter, 48 of those hours must be from 3pm - 7am, one Friday and one Sat per month. And one winter/one summer holiday. Winter holidays are Christmas Day and New Years Day. Summer holidays are Memorial Day, 4th of July, Labor Day and Thanksgiving. Pay is around $30 or so with no benefits. Some facilites will pay more if your willing to work more units. Some places have their own "in house" pool, Oakwood and St Joeseph-Mercy have both gone this route, you can work at any of their facilites, scheduling is done through a centralized system where you plug in the days you are available to work from the needs list, thus allowing you to control your schedule. If you are cancelled for a shift it generally applies toward your commitment, if you sign up for a 8hr midnight shift and get cancelled that 8 hours comes off your 48 hour commitment even though you did not work the shift.
The huge downside to these types of positions is that if you need the money to pay your bills, it is a gamble with the cancellation thing. On the up side, if you are just supplementing the family income, it allows you greater time with your family being that in most instances you are in control of your schedule.
Hope this helps!