Update: It also requires that the window manager have compositing enabled for the translucent effect to work, otherwise it will appear solid black in those sections.
Ksnip is a Qt 4 based Linux screenshot tool inspired by the Window Snipping Tool.
Latest ksnip version contains following features:
Taking Screenshot of a custom rectangular area that can be drawn with mouse cursor.
Taking screenshot of the screen/monitor where the mouse cursor is currently located.
Taking screenshot of full screen, including all screens/monitors.
Taking screenshot of window that currently has focus.
Take screenshot with or without mouse cursor.
Customizable capture delay for all capture options.
Upload screenshots directly to imgur.com in anonymous or user mode.
Drawing on captured screenshot with two different tools, pen (opaque) and maker (semi-transparent).
Drawing two shapes ellipse and rectangle, with and without fill.
Customizable color and size (thickness) for drawing tools.
Writing text on screenshots, with customizable font, size, color etc.
Any drawing can be moved around without deleting it by simply dragging it to new location.
Cropping captured screensshot, including any drawing on the captured image.
Command line support, for taking screenshot and saving it to default location, filename and format.
Customizable default location, filename and format for saving new screenshots with wildcards for Year ($Y), Month ($M), Day ($D) and Time ($T).
Print screenshot or save is to prf/ps.
Undo/Redo for paint and crop operations.
Smooth out free hand pen and marker lines (can be disabled in settings).