Kumquats are tiny citrus fruits, just about an inch long. The rind is quite sweet, the fruit inside is eye-poppingly sour -- so eat them whole to get a sweet-sour taste explosion! (I love kumquats, can you tell?)

Image here: http://www.takesavillage.net/2012/02/14/custom-kumquat-salsa-make-it-yours/ They're about 1-2 long, lots of seeds, but make fabulous marmalade, etc.

Sweet-tart citrus, looks like a miniature orange, entirely edible (including skin).

A small kind of sour orange,I think...