http://sqlrelay.sourceforge.net/index.html
It has a native TCL API and supports most popular databases.
One problem: It doesn't have production ready support to run against databases on Windows, but for the lucky people who use Linux/Unix, it seems to be an excellent solution, while TDBC is being developed.
The TCL part of the db connection will work just fine on Windows connecting to sqlrelay running on Linux/Unix (which, in turn, can connect to the database running on Windows :) ). What it doesn't have is production ready support (according to developers) for sqlrelay running on Windows.