Jing is designed for instant computer screen movies and image captures - a small yellow bubble floats on edges of your desktop - hover your mouse over the bubble and click the capture button.
You can select any rectangular areas of the screen, decide whether you want a static screenshot or a movie, and hit the record button - the graphic is saved as a PNG image while the movie will be saved as a SWF Flash file. During the record process, everything else is dimmed which I think is a very good design.
Related: The Best Screencasting Tools
You can either leave the recording on your hard-drive or upload them to Screencast.com via Jing itself. [Screencast.com is like the YouTube of Screencast videos]
The most useful feature of Jing is history (quite similar to Plasq Skitch) - every movie or screenshot captured via Jing is always accessible through the history window as a thumbnail. Deleting a capture deletes it from your History as well as Screencast.com if it has been shared.
While the Jing installer is a mere 4 MB, you have to installed the .NET 3 framework to use this sofware (and that weighs around 28 MB.)
If you don't own Camtasia, you can consider Jing for creating short screencast videos in Flash which can then be embedded in blogs or shared online through Blip.tv (yes, Blip accepts Flash files).
While there are no hints if Jing will always remain a free utility, it does signal that the Mac Screencasting community may soon be blessed with a Mac version of Camtasia and SnagIt.
Related: Screencasting Software Guide, Screen Capture Software