Even better might be highly-seasonal locations with large fleets that sit mostly idle in the off-season. A prime example is ANC, where most rental companies have a fleet of around a thousand cars in the summer and probably couldn't even keep 200 on rent in the winter. They do some downsizing in the fall but it's hard to get rid of 800 cars, so even with a fleet half the size of their summer fleet, the utilization rate is in the toilet and, consequently, rates are, too, as they try to do anything they can to steal business from their competiors and just keep some cars on the road.

Corporate-owned seasonal locations in the Lower 48 won't see this as much because it's easier to move cars around the country (i.e. the Florida and Arizona drive-outs, etc.). I'd expect franchises in the Mountain West region, like the Hertz franchise for the Dakotas/Montana/Idaho/eastern Washington, Overland West, to see this effect a bit, though I haven't checked specifically.

Weekends in super-heavy business travel markets (DC is a prime example) can be pretty good for low rates (and cheap hotel stays, too).

Otherwise, it's just the ebb and flow of supply and demand that produces these super-cheap rates. What you’re looking for is a combination of low demand, high supply, and a lot of competition. Hard to predict and hard to find but nice when you stumble upon one. The topic of searching by rate rather than by location came up here recently: Renting for fun