Hi! I have an iMac and a MacBook Pro. On both, I use grabbox, which takes screenshots and uploads them to the public directory in dropbox. On my iMac, however, screenshots are ALSO being saved on the desktop ... Why is this???
OS X's Preview.app has had this functionality integrated since Leopard. You can get to it, by selecting Tools->Annotate or View->Show Annotations Toolbar in the menu bar.