ksnip
ksnip the cross-platform screenshot and annotation tool
...Global hotkeys for capturing screenshots (currently only for Windows and X11). Tabs for screenshots and images. Open existing images via dialog, drag-and-drop or paste from the clipboard. Run as a single instance application (secondary instances send cli parameter to primary instance). Pin screenshots in frameless windows that stay atop other windows. User-defined actions for taking screenshots and post-processing. OCR support through plugins (Windows and Linux/Unix). Many configuration options.