Shutter has the most common features you would need from a screenshot program. With Shutter you can add text, shapes, arrows, step markers, blur, censor, and even add effects.
|A screenshot of Ksnapshot inside Shutter taken by Ksnapshot. Screenception?|
The main reason I find myself needing to edit a screenshot is to blur personal information. Shutter gives the option to blur or sensor.
Adding text is easy and elegant.
One feature I really like is being able to add auto incrementing step bubbles to your screenshot. This is very useful for technical how-to tutorials.
Shutter also offers a handful of filters if you want to go crazy.
Another nice feature is that Shutter can be added and used from the taskbar.