If you pop into the menu of the Canon 5D Mark III (or 1DX when available), you can change the shutter button to Start and Stop video recording. This feature wasn't available for previous Canon DSLRs (unless you have Magic Lantern). To start and stop video on previous Canon cameras, you were required to use an infrared remote, or more expensive USB remotes.

Now that this feature is available in the new Canon cameras, you can use a very basic corded shutter remote to initiate video recording. If you're looking for a small and inexpensive trigger to place next to the handle of your rig or along the pan bar of your fluid head, something like these basic shutter remotes will do. They are much smaller than my bulky time lapse remotes (seen below), so I thought i'd grab one of the smaller basic remotes just to place next to the handle of my DSLR rig.

(Above) Bulky Timer Remote - Intervalometer

Canon's RS-80N3 Remote Switch isn't very different, and will run you about $45 dollars. These cheap ones are also handy for doing long exposure photos so that you don't shake the camera and they only run under $4.00 dollars on eBay (click here).

Canon 5D Mark III Shutter Remote Start / Stop Video

There's also a similar version available on Amazon for $4.68 + Free Shipping

Remote Shutter Release Cord for Canon EOS 5D, 50D, 40D, 20D, & 10D Digital SLR Cameras