my lawn

The small mounds could be made by the Earthworm.
eg. worm casts.
Scatter the casts with a besom or a supple bamboo cane when dry.
Remove all mowings from the lawn since their presence encourages worms.
There is no pesticide for the amateur to use.

I always thought that worms are good for the garden, maybe there are some types that are damaging to lawns, not really sure, lots of worms is usually a good sign if you ask me. are you sure that it is worms? they do leave little wiggly piles of mud on the grass, but never found them to be a problem. but if it is mounds that you have I think it is more likely to be ants. ant hills can be a problem, i usually just pore water in them, and it solves the problem.

Some of my friends have reported infestation of "Dew Worms" which leave mounds all over the lawn. They are nearly impossible to get rid of! Like Fairy Ring...if you have Dew Worms...learn to love Dew Worms.!
Google search "Dew Worms" for lots of info on how to minimize the damage. Hope it is ants as Majeekahead suggests : )