Is there a MS Excel API/SDK? I just noticed a Excel plugin which was a dll
file. I am wondering whether it's possible for one to create his own plugins
for Excel and how to go about it?

Also, is it possible to link in Excel with another application, say a 3rd
party program. So that Excel worksheets and cells correspond to values
stored in a 3rd party program's database.

Is there a MS Excel API/SDK? I just noticed a Excel plugin which was a dll
file. I am wondering whether it's possible for one to create his own
plugins for Excel and how to go about it?

I could possibly be way off base here, but I think that you need Microsoft
Visual Studio Tools for Office to create something like you're talking
about.

Also, is it possible to link in Excel with another application, say a 3rd
party program. So that Excel worksheets and cells correspond to values
stored in a 3rd party program's database.

If the Database is ODBC compatable (Many of them are) then it would
certainly be possible. It would take a little bit of VBA, but not much.
You would first have to setup the Database as an ODBC data source
(odbcad32.exe). Actually, I don't even think you have to do that, you just
have to create the database connection within your code.

Lydon Bergin
MCP, MOS