Many people know about Find Usages (default shortcut Alt-F7), which find usages of element under caret. Some people known about Highlight Usages (default shortcut Ctrl-Shift-F7), which places highlighting markers in current file on element usages. However, ReSharper can highlight usages of different things as well.
Usages of Namespaces
Position caret on "using" directive (Imports in Visual Basic) and invoke Highlight Usages command. ReSharper will highlight all symbols which depend on namespace in question.
Usages of Expressions
Select expression in code and invoke Highlight Usages command. ReSharper will highlight same expressions in other places in code.