I was thinking about a piece of my camera gear that I couldn't be without.. Sure, the Camera body is crucial to photography, as are the various lenses that I have (or wish I had!) but let's go beyond those, something that I use ALL the time that I take for granted that I couldn't be without, it's an add on, it's not something like "the shutter button" or something daft like that...
Canon BG-E6 Battery Grip for Canon 5D Mark II Digital SLR
It's handy for the extra power that having two batteries gives, it's great for portrait oriented shots without having to bend your arm around like some sort of odd contortionist, it's equally at home being used as a thumping device for the heads of would be camera thieves!... Yep, it's my BG-E6
Not only does it give you more buttonage than you could want, but it has an insert that lets you shoot using AA batteries, and it looks cool, too... Try a grip out today, I'm pretty sure you won't be disappointing.