Recommend Free Database to use for commercial purposes in .net app

Depending on the size of the project you might be able to get by using SQL Express Edition.

Firebird - nice, scalable open source database with one of the best freeware .NET integration.

PostgreSQL is licensed under a BSD license, which makes it an obvious choice if your product is commercial. That, and it's a very good database management system.

SQLite also has a friendly licesnse, but it is very different from a traditional SQL DBMS.