Ec2-delete-registered-snapshot is a project mainly written in Ruby, it's free.
Simple timesaver ruby script for deleting registered snapshots
This is a simple timesaver script for deleting registered snapshots by first unregistering the ami and then deleting the snapshot. I use it for the EBS based backups that I create by another script.
ec2-delete-registered-snapshot snap-12345678
The ec2-descripe-snapshots output expected from this script is as follows: SNAPSHOT snap-12345678 vol-90abcdef completed 2010-07-27T21:52:51+0000 100% 266627911898 15 Created by CreateImage(i-87654321) for ami-abcdefab from vol-90abcdef