FxCop (new homepage) & StyleCop
FxCop: Provides static assembly analysis.
StyleCop: Provides source code analysis.
Both can be integrated into visual studio by editing your .csproj files you include tasks to run the analysis.
FxCop:
Can be integrated by calling the command line FxCopCmd tool from a post build event. Add something like this to your .csproj file. We include a common custom dictionary with our analysis too. You can miss that out if you don't want it.
<PropertyGroup>
<PostBuildEvent>"%25ProgramFiles%25/Microsoft FxCop/FxCopCmd.exe" /file:"$(TargetPath)" /searchgac /console /dictionary:"$(SolutionDir)....\Tools\FxCop\Config\CustomDictionary.xml"</PostBuildEvent>
</PropertyGroup>
StyleCop:
This is done by importing the StyleCop targets
<Import Project="$(ProgramFiles)\MSBuild\Microsoft\StyleCop\v4.3\Microsoft.StyleCop.targets" />
(FxCop is built into VS2008 team edition now, it's called "code analysis")
[Edit: FxCop has a new home now and the latest version (1.36) can be downloaded here. there is also talk of it being built into vs2010, but no mention of which level you will have to have to get it.]