I personally use JTDS and InetSoftware's JDBC Driver - both have been quite stable..
If you want a MS sourced driver, then you might want to try out the new Beta Driver out for MS SQL 2000, and 2005
http://www.microsoft.com/sql/downloads/2005/jdbc.mspx it's not based off that DataDirect POS anymore..
Support for SQL Server 2000 and SQL Server 2005
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Support for JDBC 3.0
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Improved performance
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Improved XA transaction support
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BLOB and CLOB support
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Integrated authentication support
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Updateable result sets