Custom Node Access

For Drupal 6 (might work for D7?)
The Custom Node Access module provides hook_node_access_custom().

Add custom access callbacks to node/[nid]

Example:

function mymodule_node_access_custom($node, $op, $account) {
  // Grant random access
  return rand(0,1) ? TRUE : FALSE;
}

Credits

This module was sponsored by Berlingske Media A/S