I was doing a little digging for some old emails recently and realized that it's been over a year that oss.sonatype.org has been available for free Maven repository hosting of Open Source projects.
The first was plexus, naturally since Maven and Nexus are both currently built on top of plexus.
We now have over 81 projects actively using the system to host, stage and sync their artifacts to Central. The repository is hosted on a machine at Contegix that is racked up next to the Maven Central repository, making it possible to sync new releases over hourly, not to mention that it is also extremely reliable.
If you have an Open Source project looking for a better way to stage your artifacts and promote them to Central go here to get started. We'll even help you migrate your existing artifacts.
We also provide free Nexus Professional licenses to projects or forges that would rather run their own system. You may request a license by filling out this form: http://sonatype.com/products/nexus/professional/oss