Yes, there are quite a few per diem pilots working.   

The position is what it means:  Part-Time

There are no union benefits, no dues, no grievance protections.  You work when you want to, and promise to do at least a few shifts a month.   Some work many locations, flying across the country here and there, others stay just local.  You are reimbursed for travel and expenses over and above your daily rate.   You are responsible for your own medical/401k//dental.  You generally choose when and where you wish to work, and often do lots of maintenance runs and "ferry" flights (repos).  Some are Part 91 only, others are Part 135 qualified.  Many are dual aircraft qualified.

Its a local/regional thing.  If there is a need in region, they will have a few Part 91 pilots in their hip pocket.