The first question you need to answer is whether you want a copyleft license[1], that requires all derivative works to be open sourced as well, or a more liberal license that allows one to use your code in a proprietary application.

If you are OK with a liberal license you probably want to pick either the BSD[2] or MIT[3] license. For example, Django uses a BSD license.

If you want a copyleft license you probably want to use GPL[4] or LGPL[5]

  [1] http://www.gnu.org/copyleft/
  [2] http://www.opensource.org/licenses/bsd-license.php
  [3] http://www.opensource.org/licenses/mit-license.php
  [4] http://www.opensource.org/licenses/gpl-license.php
  [5] http://www.opensource.org/licenses/lgpl-license.php