I'll add few comments and bits of historical information.
The 1-ph 3-wire 120/240V system with grounded neutral has long been the norm for residential and light commercial services where 3-phase is not required. One thing to keep in mind is that in the typical U.S. residential neighborhood you will find a larger number of smaller transformers for a given number of homes, each xfmr being single phase and feeding just a few houses. It's something of a contrast to the U.K. system where as soon as you get to any sort of built-up area you find a big 3-phase xfmr feeding 240/415 out around the whole neighborhood. It makes sense given the overall greater spaciousness and lower density of houses per acre.
Historically, 3-phase supplies in the U.S. tended to lean quite heavily toward delta arrangements, with both 240V and 480V delta being employed. Some delta supplies in earlier times were ungrounded, others were corner-grounded. I understand that in commercial applications it was once fairly common for a building to be supplied with 240 or 480 delta for 3-phase equipment plus a separate 1-ph 3-w 120/240V for lighting and other general-purpose loads.
The 4-wire delta came about as the ideal way to provide 3-ph 240V power to the existing delta configuration but also allow for 120V loads to be run from the same service. As noted already, it has the mid-point of one secondary winding grounded and brought into the building as a neutral, so you can connect any of the following:
1. 1-ph 120V loads, A-N or C-N
2. 1-ph 240V loads, A-B or A-C or B-C
3. 3-ph 240V loads between A-B-C.
The B-phase being at 208V to neutral/ground is what led to the various names of high-leg delta, wild-leg delta, bastard-leg delta, and red-leg delta, the latter from the common practice at one time of using red to identify the B-phase with its higher voltage (although since the early 1970s, orange has been the recommended color).
Although there are still delta services in use, 4-wire star/wye has gradually become the norm for newer services over the years. 120/208 is the lowest voltage system, common for light commercial applications. Where greater power is required 277/480 wye is used, the seemingly "odd" figure of 277 volts coming from the fact that it was intended to provide compatibility with older 480V delta services, seeing as existing 480V delta motors could simply be transferred directly to a 277/480 wye system. Many 3-ph motors were/are built with split stator windings so that each may be strapped for series or parallel running, thus allowing the same motor to be used on older 240 delta supplies or on the 480V of a 277/480 wye system.
Industrial buildings supplied with 277/480 will often use 277V to feed single-phase loads directly (e.g. heating & lighting), but will have a step-down xfmr to provide a 120/208 system as well. Different color coding is generally used to identify the different voltages.
120/208 wye is also commonly supplied to apartment buildings, each flat within the building then getting its supply as two phases plus neutral (and obviously with some flats getting A & B, some getting B & C, and the rest getting A & C to balance the load). Each individual apartment can then use the same type of distribution panel as would be used for a regular 120/240V single-phase supply, but obviously with only 208V available between the two "hot" legs instead of 240V.
Fixed electric heaters and similar appliances are often dual-rated for 208 or 240V operation, obviously with a slightly lower wattage at 208V. Others are dual-rated for 240 or 277V operation. For the odd places where the difference between 208 and 240 is a problem, auto-transformers can be employed for the specific load.
Over the border in Canada they tend to use 347/600V wye for higher-powered industrial supplies instead of the American 277/480V, but otherwise it's similar.
