The full version of Microsoft SQL Server Management Studio can be installed WITHOUT installing the server, and it can administer a SQL Server Express Edition.
Here's a tutorial (with screenshots) on how to do this: SQL Server 2008 Client Tools Installation
About SQL Server Management Studio Express Edition (from Wikipedia):
Microsoft has also introduced a free edition known as the Express
Edition. As with all of Microsoft's "Express" products, this is
downloadable as a standalone tool or integrated with a SQL Server
Express edition at no charge. The tool's limitations lie in the fact
that it cannot manage SQL Server Analysis Services, Integration
Services, Notification Services, Reporting Services, or SQL Server
2005 Mobile Edition.