That looks similar to the volume indicator in my cinnamon but it's not actually for the volume by the looks of it, the size shape and position's the same but I don't know what you'd call the one on your desktop. I didn't know there were any other notifiers like this for anything else, be interesting to find out what it is . . . . . sorry i can't help though !
Lenovo ThinkPad T61 Linux Mint 12 Lisa & Mint 14 Nadia (on different HDDs) 2x Intel Core 2 Duo CPU T7500 @ 2.2GHz/ Kernel Linux 3.0.0-12-generic (i686) Desktop Environment - Cinammon 1.6/ Intel GM965/GL960 Graphics Controller.