As nearly everyone knows, open source software is a low cost alternative to proprietary software. For example, the open source Linux operating system is commonly seen as a low cost alternative to Microsoft's Server 2003 operating system, or Sun's version of Unix. The popularity of open source is seen in the fact that today the largest market share for web servers is held by the open source Apache system.
The availability of the source code and the right to modify it is very important. It enables the unlimited tuning and improvement of a software product. It also makes it possible to port the code to new hardware, to adapt it to changing conditions, and to reach a detailed understanding of how the system works. This is why many experts are reaching the conclusion that to really extend the lifetime of an application, it must be available in source form. In fact, no binary-only application more than 10 years old now survives in unmodified form, while several open source software systems from the 1980s are still in widespread use (although in many cases conveniently adapted to new environments). Source code availability also makes it much easier to isolate bugs, and (for a programmer) to fix them
The right to redistribute modifications and improvements to the code, and to reuse other open source code, permits all the advantages due to the modifiability of the software to be shared by large communities. This is usually the point that differentiates open source software licences from ``nearly free'' ones. In substance, the fact that redistribution rights cannot be revoked, and that they are universal, is what attracts a substantial crowd of developers to work around open source software projects.
The right to use the software in any way. This, combined with redistribution rights, ensures (if the software is useful enough), a large population of users, which helps in turn to build up a market for support and customization of the software, which can only attract more and more developers to work in the project. This in turn helps to improve the quality of the product, and to improve its functionality. Which, once more, will cause more and more users to give the product a try, and probably to use it regularly.
There is no single entity on which the future of the software depends. This is a very common concern with proprietary software. Let us say that a company uses a software product, and relies on the software manufacturer for upgrades and continued development. If the software manufacturer closes doors, or decides to discontinue development of the product, no one has the right to take the program and continue development on it, effectively killing its usability in the market. This has happened many times, and this problem is amplified by the recent mergers in the software market, that usually lead to ``cannibalization'' of some software product to allow just one or two to get to the market. Open source software effectively protects against this, because if the group or company that originated the code decides to stop development, it is always possible to fund another software group to continue the maintenance and improvement, without legal nor practical limitations.