DB2 Express-C is completely "free" to download, develop, deploy, test, run, embed, and redistribute.

http://www-306.ibm.com/software/data/db2/udb/edition-expressc.html

DB2 Express-C, a version of DB2 Universal Database Express Edition (DB2 Express) for the community, is a no-charge data server for use in development and deployment. Providing the same core data server features,  development interfaces as well as system limits as DB2 Express in a smaller package, DB2 Express-C  offers a solid base to build and deploy all applications including: C/C+, Java, .NET, PHP, and more. DB2 Express-C is available for Linux and Windows running on 32 or 64 bit hardware with up to 2CPUs and 4GB of memory.