Mac: If you have a newer Mac with both integrated and discrete graphics on the board, it can be difficult to tell which card is operating, or whether both graphics cards are working properly. gfxCardStatus is a menubar utility that lets you see, and switch cards yourself if you wish.
The beauty of switchable graphics in recent Macs is that you can stick to the embedded graphics for lightweight tasks and longer battery life, but seamlessly bump over to the discrete graphics card when performance is needed or you're firing up your favourite game.
Once installed, gfxCardStatus lets you see which card is currently in use, what dependencies there are on the card-as in what's currently taxing it-and even lets you toggle between discrete and integrated if you want to. You can even tell your Mac to only use one and ignore the other. It's not always a good idea to do this, but if you're having graphics issues, it could be a useful troubleshooting tool.
gfxCardStatus is free (donations are accepted), and supports all versions of Mac OS, including Lion.