Ok, as you can see, I try every little gadget at least, once. Given my desperation to improve my putting, I figured it was worth a try.
I saw this at Sharper image for $50. I saw similar gadgets giving 'reads' of the greens.
http://www.thegreenreader.com/
http://www.exelys.com/
I even tried 'Plumb bobbing' which I think is a mix of voodoo and chance, personally.
Anyways, this digital level gives a degree of slope from 0 to 9.9 degrees in 8 directions. Based upon the distance from the hole, you make the adjustment of your aim based upon the degree of break. It is lightweight and has a faux leather case that can attach to the belt.
Here is the link to the manual to get an idea how it works and what it does:
http://www.exelys.com/icons/documentation/bmmanual.pdf
I have tried it a couple of times while just practicing on the putting green (after visual inspection). It has helped me 'train my eye' for putting just in the few times I have used it. It is not legal for play during tournament use. According the website caddies have used it to map the greens prior to tournament play (they make maps of the greens to show the breaks). Would I continue to use it after learning to trust my eye? I don't know. I will given an update after more applications of this particular device.
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Macgregor Mactech NVG2 11.5
Macgregor Mactech NVG2 Draw 3W

Mid Rescue 3H

4i-PW, MX-900

SW, 56*
Eidolon GW, 52*

LW, 60*
Scotty Cameron Platinum Bullseye Blade