Most Useful Attributes

Only a few attributes get compiler support, but one very interesting use of attributes is in AOP: PostSharp uses your bespoke attributes to inject IL into methods, allowing all manner of abilities... log/trace being trivial examples - but some other good examples are things like automatic INotifyPropertyChanged implementation (here).

Some that occur and impact the compiler or runtime directly:

• [Conditional("FOO")] - calls to this method (including argument evaluation) only occur if the "FOO" symbol is defined during build
• [MethodImpl(...)] - used to indicate a few thing like synchronization, inlining
• [PrincipalPermission(...)] - used to inject security checks into the code automatically
• [TypeForwardedTo(...)] - used to move types between assemblies without rebuilding the callers

For things that are checked manually via reflection - I'm a big fan of the System.ComponentModel attributes; things like [TypeDescriptionProvider(...)], [TypeConverter(...)], and [Editor(...)] which can completely change the behavior of types in data-binding scenarios (i.e. dynamic properties etc).

[DebuggerDisplay] can be really helpful to quickly see customized output of a Type when you mouse over the instance of the Type during debugging. example:

[DebuggerDisplay("FirstName={FirstName}, LastName={LastName}")]
class Customer
{
    public string FirstName;
    public string LastName;
}

This is how it should look in the debugger:

Also, it is worth mentioning that [WebMethod] attribute with CacheDuration property set can avoid unnecessary execution of the web service method.