What tools can I use to test and ensure quality in C# code?

First, you need to define your priorities in terms of code quality, and specific metrics your code needs to meet. Do you want to detect and prevent code duplication? Code complexity? Or simply ensure a coding standard is followed?

Once you have your objectives and metrics planned, you could use these tools (which I've used on a past project):

• Simian (http://www.redhillconsulting.com.au/products/simian/), to detect and prevent code duplication.
  
• RSM (http://msquaredtechnologies.com/m2rsm/rsm_demo.php) to detect code complexity and meet the requirements for function size and cyclomatic complexity
  
• NCover (http://www.ncover.com/) to indicate what areas of code are being exercised by unit tests, ie. your unit test "coverage"
  
• FxCop (http://msdn.microsoft.com/en-us/library/bb429476.aspx) to ensure the code meets Microsoft .NET Framework design guidelines

Finally, it's important to plan time upfront in the project to tweak these tools so that they output just enough information that will be useful to the development team. FxCop for example can be quite verbose by default, and making sense of its output can be a challenge. The point is to provide an automated support to your team that they fits their needs - the extra days you invest to get the tools working just right will pay off in the long run.