Also, Realtor.com has a cool Rental app that lists rentals available.

A listing agent has access to a showing app that makes scheduling showings easy, tracks who was at the property when, and anyone visiting the property is with a buyer’s agent that has passed a background check, follows strict showing guidelines, is insured, is regulated by the real estate commission, and risks losing their license or being fined if anything goes awry during the showing. 10.

There are some free apps out there but I would not rely on them.

When you post an image on a website or app (that you don't own) generally has a "Terms of Service" that give the owner of said website/app full rights to use your information in any way they wish. 

Picture a storage facility in which renters can lock and unlock their units using an app.

None of them really met our usual criteria but we decided after holding their apps for a while that if we got no better ones we would pick out the most qualified unqualified tenant.

What are some good and inexpensive/user friendly apps to help me manage my rental property?

Would love an app or a tool that I can reference quickly to determine what different properties would rent for.

Besides BP, what websites or apps would you recommend?

I see that there are several apps available that allow the user to accept calls and texts and even call out using a specified business phone number.