This is my first experience with contributing to an open data, so I tried all three:
It was very easy to participate or decide to participate with any of the projects. SciStarter required a signup, which was a slight deterrent. It would be nice to be able to sign in with an existing account. One of my tasks was to transcribe a museum image. I found this to be relevant to my current position. I was even inspired to begin the creation of a similar project of my own.
- Open your data with ScraperWiki (scraperwiki.com)
- Project OpenData Adds OpenData Implementation Guide (gisuser.com)