I would not be able to live without my tools from Red-Gate (http://www.red-gate.com/) but if you cannot afford it, maybe you can try the new "Open DBDiff" project which is freely available (including source code) from http://www.codeplex.com/OpenDBiff.

I haven't tried/tested that tool myself so I cannot comment it.

Copied from the web site:

"Open DBDiff is an open source database schema comparison tool for SQL Server 2005/2008. It reports differences between two database schemas and provides a synchronization script to upgrade a database from one to the other. I didn't found any tool like this 100% open source for SQL Server. It does not depend on any privative tool or component. I do its work without resorting to SMO."