Agree with Stephan2nd: A very handy app. With these screen capture applications, it is always asked "Why not use the built-in Command-Shift-3 or 4". Well, that built-in utility can copy to the clipboard instead of a file by adding a control key (e.g. Control-Command-Shift-3 or 4). But, the built in routine will not let you copy the clipboard to a file. Grabit does by "Grab image on Clipboard", then saving the floating image to a file.
The ability to grab a screenshot and have the image float above everything else is a wonderful thing; you don't have to go look at a file to see if you capture exactly what you wanted. You copy an image from a browser window by Control-clicking on a image and using the drop down menu to Copy, then use the Grabit command to make a capture from the Clipboard.
Finally you can do multiple screen captures of various images and have them all float in full size or thumbnail style like Stickies text notes. You can also show or hide them with a Grabit menu command.
One thing that I would like to see is to have a library window that holds all the captures, say a hundred or so. SnappyApp is one example of a screen capture utility that does this.