I use those connectors for battery connections alot at my work. They are great for this application. The SB50 is what I mainly use, the nice thing is that they are very durable, easily repaired, and self polarizing in that they can only be connected one way. They come in different colours, which only mate with the same color. They are 2 pole, good for 50 Amps.
They are, like you say very common, especially in battery / power connections. They come in sizes up to massive forklift battery disconnect with pull handles.
Others that I use for lower power ( under say 10 Amp ) are the ITT - Cannon "Sure-Seal" plugs that have a water-tite rubber boot similar to a spark plug boot. They can be configured any way you like.