The Eurorack Standard

Many early synthesizer modules had modules with height in integer inches: 11" (e.g., Roland 100), 10" (e.g., Wavemakers), 9" (e.g., Aries), 8" (e.g., ARP 2500), 7" (e.g., Polyfusion, Buchla, Serge), 6" (e.g., Emu) and had widths in 1/4" inch multiples.
More recently it has become more popular to follow the standard 19" Rack unit systems like the 6U (Wiard), 5U (8.75" e.g., Moog, Modcan), and 4U (e.g., Serge).
Two rack 3U unit systems in particular are notable: the Frac Rack system (e.g., Paia) and the similar Eurorack system.

Manufacturers using the Eurorack format include Doepfer, Analogue Systems, and Cwejman in Europe, and Plan B, Livewire, and Harvestman in the USA.

The Eurorack format uses modules of 128.5mm (slightly over 5") height.
Horizontal width is measured in "horizontal pitch", where 1 HP = 5mm (slightly less than 0.2"); most manufacturers use a horizontal unit definition of either 1U = 4HP, or 1U = 6HP.
3.5mm phone jacks are used for interconnection of audio and voltage control.

The Eurorack format needs ±12V power (most other current formats use ±15V), in addition to +5V required by some modules.
Unfortunately, the power connections are not completely standardized yet; the format uses ribbon cables for power and a two-row ribbon cable connector containing either 10, 12, or 16 pins.
The 16-pin connector uses some of the extra pins to distribute control voltage and gate signals from a keyboard to the modules via a common case connection.

More info at http://en.wikipedia.org/wiki/Modular_synthesizer,
http://electronicmusic.wikia.com/wiki/Eurorack, and
http://www.synthtopia.com/content/2009/01/31/eurorack-modular-synthesizers/ or http://www.modularplanner.co.uk/ for suggestions.