For home processing, and some specialist commercial processing for individual films, or very small batches the developing tank is the common option. All work on the principle of the film being inserted around a pair of grooved spirals set apart for the width of the film.
The spiral, or spirals where a tank takes more than one are inserted in the tank, in the dark but once the tank is sealed chemicals for processing can be added in daylight the tank only being opened once the film is fixed.
Tanks are generally plastic, although stainless steel tanks are also available. Generally each spiral takes one film and where a multiple of films are to be developed larger tanks are used to accomodate them.
Generally available today are single tanks for 35mm films, tanks that will take 2 35mm film or one 120, or 127 film. While 127 film is not very common most spirals can be adjusted to accomodate that size. Larger tanks taking more spirals are available.
There are also tanks available which can be loaded in daylight. A handle on the side of the tank is used to wind the film sealed within the tank. Generally they work well for 35mm film, but version for 120 film can at times be problematic.
In the past tanks were availbale for other film sizes, including one that could accomodate cine film.
Details of each tank can be found from the links below: