Smart tags appear in Visual Studio when the IDE thinks it can help you make quick work of a common task. I use the feature quite a bit, particularly for rename refactoring, implementing interfaces, adding using declarations, and generating method stubs.
One way to open a smart tag is to hover the mouse over a 1 pixel hotspot that lives somewhere in the vicinity of the smart tag indicator.
Fortunately, Visual Studio assigns the same keyboard shortcut to open a smart tag as Office apps: Shift+Alt+F10. The keyboard is a better approach, but Shift+Alt+F10 is one of those piano virtuoso maneuvers. Splay your fingers, point your wrists outward, and attack the keyboard with a flourish. Actually, it’s not a bad exercise to avoid carpal tunnel. On my laptop, I sometimes open the smart tag, I sometimes turn off the LCD.
When I went into the Customize menu to assign a new keyboard shortcut, I found there is already an alternate shortcut assigned: Ctrl+. (see Tools -> Customize -> Keyboard -> and show commands for View.ShowSmartTag).
Ctrl+. is a better shortcut to open a smart tag, but I’m still waiting for the day when I can blink at the screen and get a reaction.