I have one that we found as a hatch ling and she eats crickets and I found out they eat not only crickets, but other small insects like spiders.
Depends on species, region etc. There was a study done on saw scaled viper evolution and they found that saw scaled vipers from different regions had a different diet. Some ate scorpions as a main source of food. Others had rodents as a main source and some of them ate frogs and lizards. They grass snakes from southern Africa some times eat insects but this is not enough nourishment for these snakes. There are how ever some snakes that only eat insects like your leaf nosed snakes from Madagascar.
Depends on the type of snake but most eat small rodents others may eat frogs, lizards, toads, other small snakes...