License Compatibility
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All plug-ins *should* be licensed under a wordpress compatible license so that they can be re-distributed with wordpress. A fitting one is the GPL. Take care that all code incl. libraries shipped by the plugin are compatible with that license.

(This has [been a problem][1] and serious [point of debate][2] in the past.) 

  [1]: http://wordpress.stackexchange.com/questions/377/gpl-and-plugins
  [2]: http://wordpress.stackexchange.com/questions/5/am-i-allowed-to-license-my-wordpress-theme-under-the-agpl