You can't do it on Cloud unless you're using Google apps accounts - all other possible SSO solutions need access to your user base and you can't hook Cloud up to other user directories (yet)
Server supports it. If you're using Crowd for your Atlassian applications user base, it's not hard to enable SSO on them, it supports a couple of non-Atlassian systems, and supports addons and coding that allows you to get SSO for any other application that supports it. With or without Crowd, there are products available from many vendors which give you SSO across many applications as well (Usual disclaimer - I work for one of those vendors)
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