Software required to be open-source. This is not to be confused with "gratis" software, which is free of cost. Open-source, sometimes called "free" or "libre" software, has the original source code freely available to be viewed, redistributed and modified. Please don't use this as the only tag on a question; add a tag which indicates the functionality as well.
Open-source software (or free/libre software) is software for which the original source code is made freely available and may be redistributed and modified. Use this tag if it's important that the software is available under a free license (regardless of which free license it is — BSD, GNU GPL, Apache, etc.).
If you're looking for software that is free as in costing no money, use the tag gratis instead.
Also see: gratis versus libre.