Technical advantages of open source software

The main motivation for open source software is the social, ethical and organizational advantages of open source over proprietary software. For example, open source has a larger pool of possible contributors.

The technical advantages of open source over proprietary software are far less frequently discussed, and sometimes people are not even aware they exist. Open source gives you options that are impossible with proprietary software, though you might choose not to take those options in practice. Here's a partial list of some of those options.

These advantages apply if you're providing a service to users, but what if your software is entirely internal to a given company? Well, then you already (presumably) have access to the source code and can apply all these techniques whether the software is open source or not.