The current version of SQL Server is SQL Server 2005. SQL Server 2008 will be coming out later this year.
The prior versions are:
SQL Server 2000
SQL Server 7
SQL Server 6.5
SQL Server 6.0
SQL Server 5.0
SQL Server 4.2
SQL Server 4.0
(There are more, but this is about 15-20 years worth of history.)
The most common versions to see in production today are SQL Server 7-2005. Pretty much nothing older than SQL 6.5 is still in use.
There are also several editions of each version. They are:
Express Edition – SQL 2000 and greater (called MSDE for SQL 2000)
Workgroup Edition – SQL 2000 and greater
Standard Edition – Since at least SQL 6.5
Enterprise Edition – Since at least SQL 6.5