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Backtrack, a Linux pentesting distribution: http://www.backtrack-linux.org/
Fast, with a staggering number of attack frameworks and tools, this is popular and well-maintained.
Use it for an attacking OS - and, with VMWare or Virtualbox's ability to pass-through the host machine's USB devices, you can practice wireless security auditing with a virtual machine.
More exploitation targets: http://lwn.net/Articles/437221/
The linked article has a very good list of target distributions and VMs, along with a short discussion of each.
DVL is another project like Metasploitable: http://www.computerdefense.org/dvl/
DVL hasn't been updated recently, but this seems to be a working mirror of the most recent version (1.5).
Some sites with other useful materials:
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