Is there any application/project that provides you with a command line SQL client that will work with multiple databases and/or provides a mechanism for writing your own drivers?
Put another way, I'm looking for something like the mysql command line client or SQL*Plus for Oracle, but that's database agnostic.
All platforms welcome, but extra points for OS X/*nix approaches.
a SQL client and front-end for multiple database is Crunchyfrog, which offers a Native Database Shell. Not sure if this meets your requriement
Try sqlsharp from the Mono subversion repository. You can define new providers via a configuration file. They are loaded from .NET assemblies, so you can write your own provider if you want to, but Mono already has MS SQL and Oracle, plus postgres and MySQL provide their own .NET assemblies.
It works on any platform supported by Mono and MS.NET.