Instant shot is one of the simple yet best screen capture tool for Mac OS which comes with a lot of customization options.Capturing image from the web world is really easy with Instant Shot as the program comes with options that lets you capture, convert images & save them to Jpep, png or in tiff format.
You can capture full screen or a part of it and add fixed borders or re size the captured images, all in all this freeware utility does exactly what you need – Capture Screen with simplicity!
Download Instant Shot [For Mac]

@Nicki – Thanks for the mention but it’s a paid commercial software and I don’t think its feasible to spend money for tools when certain alternatives are available for free
I am used to the basic capturing needs. But to create a tutorial or for my presentation needs, I use Voila http://www.globaldelight.com/voila/voila_overview.html
I can capture any shape, from web, webcam. It also has huge editing options. You can share by Mail, Flickr or FTP.