Now that the Canon EOS C300 announcement has blown over, I'm more interested than ever in getting the most out of the cameras I've already got.
My first reaction to the C300 was to order an anti-moire filter for my 5D Mk II from Mosaic Engineering.
Here are a couple of cool and inexpensive additions for your DSLR kit:
Pico Dolly is a miniscule tabletop dolly. Although seemingly limited to smooth tabletops, floors, and counters, three things set this little gadget apart: 1) Tiny size, 2) Setup in "under 30 seconds", and 3) steering so that the dolly can travel an arc, similar to the effect of curved dolly track. The build quality looks great from the photos online. I ordered mine in a kit from Amazon.com, only $99.99 including the dolly, a flex arm, a clip for iPhone cinematographers, and a carry case.
Cinemover is a camera shuttle designed for use with improvised track. At under $100, it can make epic moves easily 4x the length of a typical 3' slider using nothing more than a 12' piece of lumber for track. I own a conventional dolly, but the ability to work at elevation over rough terrain and even over water -- this thing rocks, even on top of rocks.