I dunno. Laundry balls, which are meant to go in the washing machine and improve cleaning, perhaps even removing the need for soap, seem pure hokum.
These thingees for the dryer at least are plausible in being able to keep air circulating and not clump. I think it will depend on your dryer, though, and the load. Some dryers supposed do things to prevent clumping anyway, which would likely make these less effective. Plus I bet small loads, or loads containing small, less clump-prone items, show less effect as well.
