Some newer MacBook Pros have been packing dual graphics cards for a while now, with the ability to dynamically switch between them -- integrated for battery life and discrete for performance. It's often tricky to know which card is currently in use as the switch between the two is seamless. That's where gfxCardStatus comes to the rescue.
Sitting up in your menu bar, gfxCardStatus shows you at a glance which card is currently active, but more than that, it actually allows you to manually switch between the GPUs. A drop down menu provides you with three options: Integrated Only; Discrete Only; and Dynamic Switching, which is the system default that lets OS X decide which graphics card is required. If you want to save as much battery power as possible, leaving it on integrated only will ensure that the power hungry discrete GPU doesn't fire up. On the other hand if you want maximum performance all the time, then discrete only will make sure the most powerful GPU is fired up.
For most situations OS X does a decent job of managing power considerations, but if you want a little more control, gfxCardStatus will let you set up GPU power schemes to define which chip is used depending on power source.
gfxCardStatus is one of those little apps that's both useful and doesn't get in your way. You won't need it a lot, but if you're on battery power and you want to keep working for as long as possible, the little menu bar app can be a life saver.
It's freely available (donations accepted) to download from the developer's site and is Lion-ready.