Inheritance Margin by Sam Harwell adds special glyphs to virtual methods and properties in the Visual Studio 2010 editor. For interface methods it adds the I glyph and for virtual methods it adds the O glyph:
When you hover the mouse over a base glyph it displays what types implement or override this method or property:
When you hover the mouse over a derived glyph it displays all base methods that are overridden:
Inheritance Margin currently supports only C# and Visual Studio 2010. You can download this free extension from Visual Studio Gallery.