GIMP has a fantastic tool for selecting a single or multiple similar items in an image. Foreground Select Tool.

It is based on the Simple Interactive Object Extraction (SIOX) method. Essentially you loosely select the region that contains the object or objects you want to select, then highlight the items you want selected and press enter. It's pretty intelligent and there are many settings you can play with to tailor it to your image. Check out the video and see the link to the official docs after the jump.