Many open source contributor agreements (including the Alfresco Contributor Agreement) do not involve any reassignment of copyright - instead they grant the project maintainer a license to use, modify and distribute the contribution.
The Full Picture
I was recently chatting with Jennifer Venables (Alfresco's awesome general counsel) and she mentioned something in passing that I hadn't realised before, and that I'm guessing many of you may not know either. I had always been under the impression that most, if not all, contributor agreements (whether for open source projects or not) involved the individual contributor handing over copyright on their contribution to the project maintainer (for example Alfresco Software Inc., in the case of contributions to the open source Alfresco content management system). If you're like me, the thought of handing over 'your babies' to someone else is not particularly appealing, and in general I've made a point of not becoming involved in projects that require me to give up rights to my own creations.
So Jennifer's off-hand comment took me a little by surprise, and after she patiently explained how contributor agreements typically work, I'm looking at them in a more positive light. Specifically, the Alfresco Contributor Agreement (and those of some other open source projects) do not involve any reassignment of copyright - you as the creator of a particular contribution retain full ownership of the copyright of that contribution. Instead, the project maintainer is simply requesting that you license your intellectual property to them, so that they can also use, modify and distribute it - rights you've probably granted to the public anyway (at least if you've chosen one of the more popular open source licenses).
It's also worth noting that the Harmony project is an attempt by the wider open source community to try to standardise and clarify contributor agreements, as there seems to be a lot of confusion around them. Jennifer has been keeping an eye on their progress on behalf of Alfresco, as their initiative would help to clarify what is often (and definitely was for me) a confusing legal mechanism.
<disclaimer aka IANAL>Now I'm about as far from a lawyer as it's possible to get, so everything I've said here you'd be strongly advised to double check with someone who has legal expertise in this area.</disclaimer>
What I can say with certainty is that I had completely misunderstood the intent of Alfresco's Contributor Agreement and (more importantly) the legal basis upon which it operates, and that misunderstanding has prevented me in the past from contributing to other open source projects. I guess I'll chalk this up to 'sometimes, you just don't know what you don't know'!