Which Digital Level Gauge to buy?

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!

The problem with phone-based levelling apps is that the case of the phone is neither perfectly flat nor necessarily parallel with the internal gyroscope. It certainly isn't on my Moto Droid with its sliding keyboard and slightly bulged battery cover. ;)

The internal gyro may be "perfectly" level but that's of little use if the back of the phone is non-flat and/or non-parallel to the gyro.

I use a cheap but well made bubble level. The key is to use it properly. Check the spot you're interested in, then spin the level 180 degrees and check again. Reversing the level doubles the visibility of any error. Only when the bubble is centered with the level pointing in EITHER direction is the surface level (along that orientation). Never trust any level without doing this. A decent bubble level used this way (as machinists do) can produce pretty accurate results. Expensive tools are good, good technique is better. :)