We review a lot of programs every day here at Softonic and we tend to take a lot of screenshots. The tools we use to do this need to be easy to launch and offer a good set of tools.
Copernicus is a solid screen capture tool which allows you to capture parts or your whole screen. You can also record small videos. Copernicus lets you set the frame rate and decide whether to rely on memory or hard drive.
I like how Copernicus includes a preview interface for batch handling your different screen grabs. When saving you'll notice you can adjust the quality of the image, however you won't be able to change the format from JPG.
I didn't really like how Copernicus worked with F keys, which I don't find the most comfortable to work with on a Mac. However other users might find them very handy for controlling the program.