Wisps will spawn in wooded areas and you can catch them in glass jars. Once you've caught a wisp you can place it in the world to get some special perks.

Red wisps will kill plants in a 10x10x10 area.
Blue wisps will put out fires in a 10x10x10 area, although they didn't seem to work that great in my tests.
Green wisps will make plants grow faster again in a 10x10x10 area.