REP

Copied directly from ChickenBone's Minecraftforum.net entry:

The Redstone Ether Pulse or REP is essentially the redstone version of an EMP. It is a throwable device that will generate the jamming effect apon impact. A second click with one in hand will detonate an aerial REP. The resulting energy wave will prevent further REPs from being thrown for a few seconds.

Uses
As stated above, it jams wireless devices in the radius of detonation. It should be noted that one of these will kill an unarmed player, considering the player doesn't move and the tendrils hit properly.

They stack up to 16.