Sourceforge supports OpenID logins since may 2008.
Since at least 2013-10-29 their account login page does not exhibit the OpenID form anymore:
I asked in their support IRC channel, and they told me that I was the first to ask for it; nobody else noticed that they disabled it. You now have to append ?openid to the URL to get the OpenID login form:
They still support OpenID delegation and their OpenID support page does still exist. I wonder how long.