There are two versions of SharePoint Server 2010 beta available to the public:
SharePoint Server 2010 (Enterprise Client Access License features)
For organizations looking to expand their business collaboration platform to enable advanced scenarios. Use the Enterprise capabilities of SharePoint to fully interoperate with external line-of-business applications, web services, and Microsoft Office client applications; make better decisions with rich data visualization, dashboards, and advanced analytics; and build robust forms and workflow-based solutions.
SharePoint Server 2010 for Internet Sites, Enterprise
For organizations looking to create customer-facing public internet sites and private extranets using the full enterprise capabilities of SharePoint. This provides full SharePoint Enterprise functionality and no other technical limits.
Download either of them from here:
Keep in mind that you need a 64 bit environment to install SharePoint 2010 on. Check out my earlier post for the link to hardware requirements or choices for running it in a virtual environment.