Recently a user on Atlassian Answers asked about difference and relations about all of these products and that caused me to write that article considering there may be others with similar questions.
SourceTree is easier to explain, it is a free Git & Mercurial client for Mac and Windows. It is a desktop application to work on Git & Mercurial repositories. You can use it with any local/remote Git or Mercurial repository but it also integrates with Bitbucket and Github.
Stash & Bitbucket Cloud
Stash was Atlassian’s Git intranet repository manager. You had to install it your own server, it was not a cloud offering. Atlassian’s cloud offering for the same purpose was Bitbucket. This was a little confusing for the customers, so starting from ???? Atlassian renamed Stash as Bitbucket Server and Bitbucket is renamed to Bitbucket Cloud. That made Bitbucket naming standard with other Atlassian products, most of which are offered as Cloud and Server versions. There is no Stash anymore, only Bitbucket Cloud and Bitbucket Server. Note that there are some capability differences between Bitbucket Cloud & Server offerings. Most importantly Bitbucket Server does not support Mercurial but Bitbucket Cloud does, although these differences are slightly dissapearing.
What about Fisheye? Unlike Bitbucket, Fisheye is not a repository manager. It only provides web ui for your source code repositories (Git, SVN, Mercurial, Perforce, CVS). Bitbucket is a standalone repository server and provides web UI for your repositories, you don’t need to install another source code repository server. With Fisheye you can browse and search source code without Crucible add-on (paid) you can’t review source code. Although Bitbucket only supports Git, it support much more functionality like code review, merging, branch management.
What is Crucible? Crucible is Atlassian’s code review offering. It contains a lightweight Fisheye, so you can perform code reviews event without Fisheye. I personally do not suggest Fisheye and if you are not a Git shop, I only suggest you to buy Crucible for code reviews.
If your source code management software is Git, you only need Bitbucket. You don’t need Crucible for code reviews. Yes, Crucible has more advance features for code reviews but honestly you don’t need them. In Bitbucket you review commits or pull requests, in Crucible you can create reviews assign a review moderator, review deadline, add reviewers to review. Crucible keeps track of how much each reviewer reviewed in terms of coverage and time. Bitbucket has much lightweight review process, but it is enough for me. Your millage may vary.