We went with SAML mainly because of the standardization. A lot of different technologies are in use at Rackspace because teams are given a lot of autonomy. There are open source SAML libraries out there for pretty much any language, so having another team hook into our SSO solution isn't too difficult.
I can't speak for the services out there. We built on top of open source libraries to roll our own solution. The protocol is actually pretty straight-forward to implement.
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