All things considered, the built-in screen capture tools in Mac OS X isn't too bad. You have a choice of automated screen captures using the [Cmd] +  (full-screen) or [Cmd] +  (area selection or – by subsequently pressing [Space] – a selected window), or you can use the built-in Grab tool.
Both of these options are perfectly adequate, but in an ideal world you'd have the best of both worlds: the ability to take screenshots from anywhere coupled with the ability to place the mouse pointer on-screen and choose where to save your grabs. Then you might also want finer control over the name of the capture files, plus choose a different format to the default PNG on offer without having to dive into the command line.
There are plenty of third-party capture utilities out there, but most of them cost money, and don't always add much to what's already there. Before plumping for one of these tools, we'd recommend you take a look at InstantShot! This well-established utility combines – and in some cases refines - the best parts of both OS X tools, and adds in some useful tools of its own: take multiple shots at set intervals, choose between TIF, JPG and PNG quickly and easily, remove the background image from screen captures, and set a default path and filename for all screen captures, for example.
At first glance it looks like you can only capture the entire screen or selected windows – thankfully this isn't the case: choose the option to shoot inside a rectangle, then simply drag the mouse over the area of the screen you wish to capture. Full instructions for using the rectangular mode are displayed on-screen when it's selected, revealing how to modify the rectangle after it's been created (hold down [Shift]) among other options.
Version 2.5 is compatible with OS X Tiger, Leopard and Snow Leopard, but doesn't work with Lion. Thankfully after a near four-year hiatus, DigitalWaters have resumed development, and OS X Lion users can download a beta of InstantShot! 2.6 instead.