For the latest version of Bitbucket (2016+), the download link can be found in the Download menu item.
Pre-2016
First method
In the Overview page of the repo, there is a link to download the project.
Second method
Go to Downloads -> Branches -> Download the branch that you want (as .zip, .gz or .bz2). There you'll find download links for all tags. The links will be in the format:
https://bitbucket.org/owner/repository/get/v0.1.2.tar.gz
By tweaking it a little bit, you can also have access to any revision by changing the tag to the commit hash:
https://bitbucket.org/owner/repository/get/A0B1C2D.tar.gz
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