Contributions to open data do not have to be large, as long as many people are doing it. The Wikipedia is a good example, and PubChem accepts contributions of small databases too (I think). The result can still be large and rather useful, even scientifically.
The latter was recently written down in the paper Internet-based monitoring of influenza-like illness (ILI) in the general population of the Netherlands during the 2003–2004 influenza season by Marquet et al. (DOI: 1471-2458/6/242). The data was provided by Internet users via The Great Influenza Survey website. The article states that the sum of all those small contributions (anonymous website users are asked to fill out a weekly form), yields reliable data. The user is rewarded by colorful pictures, such as:
If all chemists and biochemists would add information about or properties of one molecule or metabolite to the Wikipedia each month, one or more commercial database companies will have to change their business model soon. Oh, you already can start doing this here.