Top of the list is formmail.pl

Then there are systems based on certain versions of xmlrpc.php

Finally a new contender is contactus.php. Its header is

#-----------------------------------------------------------
#-----
#----- Forms To Go v2.0.4 by Bebosoft
#-----
#----- http://www.bebosoft.com/
#-----
#-----------------------------------------------------------

Any of these scripts will be taken down, and may be cause to take down your whole website