A local bee motel. There was a wasp checking in, to. This is a rather elaborate one, offering several possibilities for wood- and cavity- nesters. (But don’t forget the ground-nesters!)
The Xerces Society has some helpful hints on building your own to encourage pollinators.
Update and caveats 11/19: these elaborate, crowded situations don’t replicate natural conditions. They turn out to be ecological traps — primed for pathogens and parasites, especially when they aren’t cleaned and maintained. Good intentions, bad results. So, don’t try this at home. Instead make habitat: give the creatures the space, plants, etc. they need.
Thanks for that link to the xerces society. Awesome!