SQL Server 2000
xp_MD5 at http://www.codeproject.com/database/xp_md5.asp

Also works in SQL Server 2005 32 bit

SQL Server 2005

DECLARE @Data varchar(MAX)
SubString(master.dbo.fn_varbintohexstr(HashBytes('MD5', @Data)), 3, 32)

Works in SQL Server 2005 32 and 64 bit

Solution found at http://www.codeproject.com/database/xp_md5.asp?df=100&forumid=32460&select=1519990#xx1519990xx