[wp-hackers] How to tell if a plugin is active?

[Ian Barton]

Wilst experimenting with plugins I often end up temporarily disabling 
them in the Comntrol Panel. Often this can cause index.php to crash with 
  "function not found" type errors:<(

Is it possible to tell if a plugin is currently active, so that I can 
write code like:

if (MyPluginActive)
   <?php myplugin_function?>
else
  print "Plugin MyPluginnot active"

Ian.