There are many groups and associations promoting their idea of the ideal scale and gauge combination. Here is a collection of the ones that I have been able to find so far.
2mm/foot
The 2mm Scale Association
3mm/foot
The 3mm Society
4mm/foot
The Double O Gauge Association
The E.M. Gauge Society Ltd.
Scale Four Society
7mm/foot
The 7mm Narrow Gauge Association
10mm/foot
The G Scale Society
16mm/foot
The Association of 16mm Narrow Gauge Modellers
7/8 inch/foot Scale
7/8's info Page
9 mm Gauge
The 009 Society
O Gauge (1.25 inch - 32 mm)
The Gauge O Guild
Gauge 1 (1.75 inch - 45 mm gauge)
The Gauge 1 Model Railroad Association
Gauge 3 (2.5 inch gauge)
The Gauge 3 Society
The National 2½ inch Gauge Association
Sacremento Valley Live Steamers