SourceTree 2.2 forces users to sign in to an Atlassian account
If you think this is pointless and annoying, please upvote this issue: https://jira.atlassian.com/browse/SRCTREE-3452
If you want to download older versions they are here (for now)
http://downloads.atlassian.com/software/sourcetree/SourceTree_2.1.dmg
https://downloads.atlassian.com/software/sourcetree/windows/SourceTreeSetup_1.7.0.32509.exe
I'm not sure if the Windows version has the same annoyances or not