It is my fervent hope that the more people learn about the unjust, political, and often arbitrary use of the death penalty, the more they will question its use, and that, in time, such questioning will lead them to understand the inherent immorality of state-sanctioned murder.
For those agnostic but unconvinced, I recommend reading Dead Man Walking, by Sister Helen Prejean. This book had a powerful effect on me, ultimately changing my conditional opposition to the death penalty to absolute.
Al Jazeera global capital punishment infographic here (interactive).
National Coalition to Abolish the Death Penalty (US)
Abolish the Death Penalty (Amnesty International)
International Commission against the Death Penalty
The Innocence Project