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.