My nomination for the best screenshot software: Shutter
Shutter is a free and opensource screenshot program with great features. You can capture the whole desktop, only a specific window or select an area that you want to capture. It has built-in edit functionality to add lines, text and arrows, a crop tool and a tool to censor / pixelize areas (e.g. to quickly hide private data if you upload the screenshot to the Internet).
Shutter comes with many plugins that allow you to apply additional effects after you made a screenshot. You can add shadows and watermarks to your screenshot, make it look like torned paper, a polaroid or a jigsaw piece or apply 3d effects. You can also import pictures into Shutter and apply effects albeit it is a bit slow with bigger files.
Screen dumps can be exported in a variety of formats, like png, jpeg, pdf, tga, xbm or bmp. You can also directly upload them to an FTP server or to imageshack.us. Uploading to Flickr requires an addtional program called Postr to be installed.
Shutter is available for all major Linux distributions, the official web site can be found at http://shutter-project.org.