Which Digital Level Gauge to buy?

yes Mauidj, there is a center hole for the gauge to fit on the platter spindle, as well as just placing the gauge on various positions of the platter itself. I have found especially with a unipivot tonearm such as the 10.5i on the VPI SSM it has made a substantial improvement both in terms of tracking and solidity of sound. there is absolutely no rocking of the tonearm whatsoever when it first touches the record or anytime during record play. check out the review of the cartridge man digital level on six moons.com, December 2005. I now consider this to be essential part of my setup toolkit when installing a cartridge, along with my mint lp protractor, digital stylus gauge, and other things. when the level is exact it reads 0.0 in all planes. I could not get this performance with a high quality carpenters level.

I use a 6" digital level from home depot about 50 bucks. Just downloaded the spirit level plus recommended above, WOW now that is killer!!!.

I agree with Dgad that android phone application "Level" is really slick.

There are over 20 Apps for levels in the App Store.
The problem with the App levels is that they all need calibrating. So who has a 100% level surface to do that on? They might get you closer than a regular carpenters bubble but from what I have read in the reviews they are not close to the accuracy of a dedicated tool.

I just downloaded a twin axis level app called Gyro Level.
It works from the internal gyroscope in the iPhone so does not need calibrating.
It aligns using gravity.
I would love to find a level table to check it on as this thing could be the $0.99 answer to the $350 question!