Run
SELECT @@version
It will say what edition you're running.
From what I recall, the editions are:
Express (free)
Workgroup
Standard
Enterprise
Developer (enterprise features, not for usage in producion system)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass