If you are an OS X Mail.app user, make use of Indev Software‘s awesome plug-ins MailTags or MailActOn, and have been holding off upgrading to Mavericks because these plug-ins aren’t compatible, there is good news and mildly less good news. Indev has been making the betas of both plug-ins available to the public so that Mountain Lion users can jump on the Mavericks bandwagon, and so they can get some feedback on the new plug-ins’ operability. Please note that standard beta software warnings apply, as stated on Indev’s own website.
“Use at your own risk! Keep a a backup of critical data.”
“These builds are not fully tested and may contain stability issues and other bugs. We will try to document known issues and fixes as they occur.”