- CostlyAxis
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- Intrepid Mythic Member
Ergo Proxy is a very good series, and is a mere 23 episodes long. There is also Black Lagoon which is 24 episodes long (excluding the 5 episode OVA). Also, you might consider Baccano! which is 13 episodes long (excluding the extended ending)
Edit: Quote me to view an even longer list of recommendations that don't necessarily meet the shortness requirement.
[Edited on 12.25.2012 4:52 PM PST]