Eurorack

Euro is a modular synthesizer format. The height of Euro format synthesizer modules is equal to three vertical units in a standard 19" equipment rack. This standard height is called "3U". Euro format modules typically use 3.5mm (1/8") connectors and plugs for patching.

Here's a list of the depth of Eurorack modules:
(courtesy of felix, from this thread)

Livewire
Frequensteiner - 1.50" (38.1mm)
Dual Cyclotron - 2.25" (57.15mm)

Plan B
Model 28 - 2.50" (63.5mm)
Model 15 - 1.75" (44.45mm) (MKII, MKI is much deeper)
Model 14 - 3.75" (95.25mm)

The Harvestman
Tyme Sefari - 2.00" (50.8mm)
Malgorithm - 1.50" (38.1mm)

Doepfer
Doepfer module depths can be found in their "planner" Excel file: Planner

Manufacturers of Euro format modules include the following: