Original Post: http://social.technet.microsoft.com/Forums/en-US/sharepointadmin/thread/ef50eb2d-7e12-4373-90c8-75832c20ac21
INSTALL:
stsadm.exe -o installfeature -filename "foldername\feature.xml"
ACTIVATE:
stsadm.exe -o activatefeature -filename "foldername\feature.xml" -url http:\\sharepoint.website.com
Note: the filename is the relative path to the Feature.xml file in C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\FEATURES.