There are currently five kinds of manometers -- vacuum gauges -- one can use to synchromize multiple throttles. The venerable bourdon tube (mechanical gauge) type, the mercury type, the water variety, the newer electronic manometers, and this type, the steel ball type. This one is sold by Suzuki, and is more compact than any of the others, and plenty accurate.

Update 2015: Since this article was written in the 1990s a lot of very good mechanical type (as opposed to fluid) manometers have become available. The Morgan Carb Tune is one of the most popular.