Openstack.org is providing APIs and open source stack in the area of IaaS, Infrastructure as a Service. So it is more suited to service providers who want to provide virtualization to consumers or more like Amazon EC2 or rackspace like offerings.
Openstack also talks about using the same open source stack in enterprise as private cloud or private IaaS. This is in direct competition with Eucalyptus stack. But I think that openstack is much more advanced and also has lot of open source community support.
Here is the link for openstack.org - Openstack.org
Here is the link for Eucalyptus - Eucalyptus.com
Happy cloud computing :)