Canada's largest union, CUPE, switched their web site to Apache, MySQL and PHP a year or so ago and won a global contest for union web sites. This year, though they face a challenge from the Teamsters, who're running IIS.
Sure, you could check Netcraft to see what they're running. Or you could check this note at the bottom of their home page: Note: Due to high Internet virus activity, we are no longer accepting website feedback via email. Please send any web feedback via U.S. Mail to the address above.
Vote for the open source site.
Acknowledgement of blatant plug: I work for CUPE.