SQL Server 2005 Editions Comparison
I found the following buried in a document (SQL Server 2005 Product Guide), but I could not locate it on the Microsoft site.  
UPDATE: The link was staring me right in the face on the SQL Server homepage!  Think of the table below as being a summary for this: http://www.microsoft.com/sql/prodinfo/features/compare-features.mspx
| Enterprise | Standard | Workgroup | Express | 
| $24,999 per processor $13,499 (server + 25 users)  | $5,999 per processor $2,799 (server + 10 users)  | $3,899 per processor $739 (server + 5 users)  | Free | 
| Fully integrated data management and analysis platform for business critical enterprise applications | Complete data management and analysis platform for medium businesses and larger departments | Most affordable and easiest to use database solution for smaller departments and growing businesses | Fastest way to learn, build and deploy simple data-driven applications | 
| Unlimited Scale and Partitioning | 1-4 CPUs Unlimited RAM  | 1-2 CPUs 3 gigabytes (GB) RAM  | 1 CPU 1 GB RAM 4 GB database size  | 
| Advanced database mirroring, complete online and parallel operations, and database snapshot Advanced analysis tools including full OLAP and data mining Advanced reporting with customized, high scale, and ad-hoc reporting Advanced ETL with complex data routing and transformation capabilities Available in native 32- and 64-bit editions*  | Database mirroring Basic ETL Standard OLAP server with Analysis Services Standard reporting with Reporting Services Data mining Full replication and SSB publishing Available in native 32- and 64-bit editions*  | Management Studio Import/Export Limited replication publishing Clustering Back-up log shipping Basic reporting  | Simple Management Tool Simple reporting Replication and SSB Client  | 
*Supports Itanium 2 and x64
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