The snare is among the first traps Geralt encounters in The Witcher 2: Assassins of Kings. Most often, he finds them a bit too late, after stepping on one.
It does, with the Alchemist skill at level 2, 40 – 100 damage to one foe, and can be gathered from the field once used. You will find many of them in the forest in Chapter 1. On Iorveth's path, some areas in the Mines of Vergen have armed snare traps.
The fact that it does quite high damage, can be placed in packs, and can be gathered and reused makes it ideal to save money and kill monsters you're having a hard time to kill by the sword.