Hacking Practice: Purposely Vulnerable Software

Vulnerable VMware/VirtualBox images from past CTF competitions:

http://ctf.hcesperer.org/25c3ctf
http://ctf.hcesperer.org/daopen08
http://ctf.hcesperer.org/eh08ctf
http://ictf.cs.ucsb.edu/images.php
http://lampsecurity.org/capture-the-flag-5 (currently down)
http://lampsecurity.org/capture-the-flag-6 (currently down)

Many more resources: Damn Vulnerable Web App, Damn Vulnerable Linux (download), OWASP's WebGoat, De-ICE, IronGeek's Mutillidae, Bonsai's Moth, and Foundstone's HacMe Video Series.

I am currently going through these myself.