From the size, I would guess a calamondin. Is the peel fairly sweet? Or possibly a chinoto (spelling?)

Looks like a calamondin but I could be wrong. You can always tell a kumquat hybrid because they have green cotyledons. It could also be a fukushu (changshou) or meiwa kumquat.
http://www.citrusvariety.ucr.edu/citrus/fukushu.html
http://www.citrusvariety.ucr.edu/citrus/meiwa.html
Show us some pictures of the plant leaves.

Calamondin or Chinotto would be my guess as well unless we can see the seed cotyledons or the leaves as morphone suggests. I'm going to lean towards calamondin as my chinotto doesn't plug that easily.

.....Alan.

Many (most?) pure mandarins also have green cotyledons. But I'd also guess calamondin.

I also guess: That's a Calamondin.

For a good determination, please do sharp pictures of the whole fruit.
Calamandins have typically truncated to depressed apex (IPGRI Index7.4.8 value 4 to 5) and have also a truncated base (IPGRI Index 7.4.7 value 3)
Helpfull were some pictures of the leaf and leaf stalk shape, because Sour Oranges have typical leaf and leaf stalk shapes, as Calamondin, too...