Just like we have volunteer firefighters in more rural areas, some police departments will allow you to volunteer. The rural department I'm at has full time, part time, and "auxiliary" (volunteer) officers. The way our state's laws are written, upon graduation of an academy you are granted a "commission" which needs to be held by a department. Regardless of pay status, once a department holds your commission you are a fully sworn officer with all the duties and responsibilities of a full-timer. There are benefits, like more freedom to try out new police tactics and ability to create different approaches to fighting crime. Downside is no pay, which I'm okay with. If your commission expires you will need to do the police academy all over again.