Chris G's method would be great, I'll have to add it to my list. Google + site's domain name would be another way. 1 search google for just the site, 2 search google for just the @domain_name.
I think the big question is though, why do want the email address?
From a pentest perspective, I could see collecting the different email addresses for trying to get possible log in names, or people in the company to try and impersonate for Social Engineering.
From a security standpoint to see if people are spoofing your comapny / found an open relay.
From a non-security related world, the only legal reason I could see doing this would be for an EECB (Executive Email Carpet Bomb). http://consumerist.com/259713/how-to-launch-an-executive-email-carpet-bomb
There are other methods, if I recall correctly, covered in Hacking for Dummies. But you really should only try to get email address for ETHICAL reasons. Spamming people is bad. Trying to get the information for just showing of is bad too.