I made a patch for supporting multiple server in active check mode.
I heard the needs in Japanese Zabbix community meeting.
Some users says they want Zabbix to support multiple server in active check for redundant monitoring.

Following is some explanation for my patch.

• The attached file is patch for zabbix-1.9 (r16152)

• "ActiveCheckServer" parameter
  I added new config parameter "ActiveCheckServer" to avoid to make needless process.
  Because active check uses one process per one Zabbix server.
  For example, if I have a Zabbix server "192.168.0.1" and add "127.0.0.1" to "Server" for zabbix_get test, the process for "127.0.0.1" is needless.
  So, I made a new parameter for active check.

• "__declspec(thread)" (Thread Local Storage in Visual C++)
  I added "__declspec(thread)" for Windows agent to avoid to refer same variable by different thread.
  Thread Local Storage is needed for Windows, because Windows agent works in thread.
  It is not needed for Unix and Linux, because the agent for them work in multi process.
  (reference) http://en.wikipedia.org/wiki/Thread-local_storage