GIMP is the undisputed king of image editing in Linux platforms, and is next only to Photoshop in popularity in Windows and Mac platforms. With a large community of developers and an even larger pool of users, it is no surprise that GIMP is very popular.
Much like Firefox, GIMP’s strength lies in its plugins, which are developed by the open-source toting community. Since the users themselves develop them, they know all the needs and conceive a plugin for everything (well, except making a coffee for you).
And what better way for me to endorse GIMP than a list of good plugins? Most of them are aimed at web-designers and photographers, the main users of GIMP. There are also some to fix common problems of GIMP.
Look at the list here.