The open-source community has organized itself in a way that tends to amplify the productivity effects of open source. In the Linux world, in particular, it's an economically significant fact that there are multiple competing Linux distributors which form a tier separate from the developers.
Developers write code, and make the code available over the Internet. Each distributor selects some subset of the available code, integrates and packages and brands it, and sells it to customers. Users choose among distributions, and may supplement a distribution by downloading code directly from developer sites.
The effect of this tier separation is to create a very fluid internal market for improvements. Developers compete with each other, for the attention of distributors and users, on the quality of their software. Distributors compete for user dollars on the appropriateness of their selection policies, and on the value they can add to the software.
A first-order effect of this internal market structure is that no node in the net is indispensible. Developers can drop out; even if their portion of the code base is not picked up directly by some other developer, the competition for attention will tend to rapidly generate functional alternatives. Distributors can fail without damaging or compromising the common open-source code base. The ecology as a whole has a more rapid response to market demands, and more capability to resist shocks and regenerate itself, than any monolithic vendor of a closed-source operating system can possibly muster.
Another important effect is to lower overhead and increase efficiency through specialization. Developers don't experience the pressures that routinely compromise conventional closed projects and turn them into tar-pits -- no lists of pointless and distracting check-list features from Marketing, no management mandates to use inappropriate and outdated languages or development environments, no requirement to re-invent wheels in a new and incompatible way in the name of product differentiation or intellectual-property protection, and (most importantly) no deadlines. No rushing a 1.0 out the door before it's done right -- which (as DeMarco and Lister observed in their discussion of the `wake me when it's over' management style in [DL] ) generally conduces not only to higher quality but actually to the most rapid delivery of a truly working result.
Distributors, on the other hand, get to specialize in the things distributors can do most effectively. Freed of the need to fund massive and ongoing software development just to stay competitive, they can concentrate on system integration, packaging, quality assurance, and service.
Both distributors and developers are kept honest by the constant feedback from and monitoring by users that is an integral part of the open-source method.