I just learned something about Subversion that I probably should have known a long time ago, as it would’ve saved me a ton of time in managing open source projects. The svn:externals property allows you to link other repositories into your own repository, thereby making it so you don’t have to manually merge third-party code when it’s updated. Unfortunately, this wouldn’t have worked when I was doing Drupal development as they are still stuck on CVS, but I think the majority of open source projects have moved to SVN by now.
The one thing lacking with SVN Externals is the documentation. The official Subversion page about it doesn’t really tell you how to use it; it just glosses over it with a quick demo. I personally found Matthew Weier O’Phinney’s blog post to be the best explanation of how to use it to its fullest extent. I’m just glad I can use it to reference TinyMCE’s repository and not have to remerge all those directories with each new release! It’s also great if you have common libraries shared among various projects in your own repository, as you can just keep the code in one place and reference it in each project.