FreeTDS is a free (open source, LGPL-licensed) implementation of
Sybase's db-lib, ct-lib, and ODBC libraries.  Currently, dblib and ctlib
are most mature.  Both of these libraries have several programs know to
compile and run against them.  ODBC is just a roughed in skeleton, and
not useful for real work.

FreeTDS will run at protocol versions 4.2, 4.6, or 5.0 and thus can
access both Sybase and Microsoft SQL-Servers.  Note: Microsoft SQL
servers do not support TDS versions above 4.2 and need to be configured
with the --with-tdsver=4.2 flag.

Experimental support for TDS 7.0 (the version used by MS SQL Server 7.0)
is now included.  It works for me but beyond that I can't promise much.
That said, TDS 7.0 support can be configured by using the
--with-tdsver=7.0 when running configure.