Pretty sure that's [Armor Staus HUD](http://www.minezone.pro/download/mods/16097-164-armorstatushud.html) (non-english site), also found on the [Minecraft Forum](http://www.minecraftforum.net/topic/1114612-172-bspkrs-mods-armorstatushud-directionhud-statuseffecthud/), which provides a heads-up display for both armor and tools with durability (by adding a swing counter).

If by chance it's not the _exact_ mod you have, ASH does the exact same thing that it does.