I'm fond of All Stays.com
Partially because I can also use it to look up a restaurant (I like to eat) or store for needed shopping.
Lots of folks don't know that many Tractor Supply Stores, Lowes & Home Depots allow overnight parking as well. ALWAYS ask the manager's permission
not a greater, security or cashier! Some towns do not allow overnight parking ANYWHERE. Buy ear plugs and lock everything down if you want to park in parking lots. Keep a cellphone turned on so that you can call 911 asap in case of a disturbance. Sam's Clubs consistently have security that patrols the parking lot at night. I feel safer in a Sam's Club parking lot plus we have parked in Sam's Clubs when ONP was not allowed in the Wal-Mart next door. I think it has something to do with the membership thing.
Many of the websites listed above depend on users keeping them up to date. If you use a site to stay overnight, then please try to add or update a listing.