a) work - this rules out www.dtheatre.com/scripts/formmail.php, the first google hit for 'PHP formmail'.
b) be secure - I'd prefer to set the destination for email in the script, not the form, for example.
c) not have too much crap attached - a 150k script for this is silly.
d) not insist on people giving an email address when they submit the form - I've hacked http://cgi-central.net/scripts/aformmail/ so it doesn't, but if people have one where I don't need to do so, then...
e) not be in a .zip file that neither pkunzip or winzip can extract from!