Believe it or not, I never heard the phrase "mudroom" until I moved to Manitoba. I believe that the phrase is rural or Prairie in nature.
In Ontario, we always referred to this as the "back porch" (if it had a chair or bench to actually sit down and take off outside articles of clothing) or the "back hall" or "back door" (no place to actually sit down, but at the very least, a large rug or piece of carpet to kick off and put those wet articles).
If one had a big front entrance/foyer/entryway, it is conceivable that they may put a seating area there, but, typically the front door of the house was seen as where one would receive formal or infrequent guests, while the occupants of the house and close friends would usually use the back door.
It is not unusual to place anything from a basic chair or bench by either entrance, but articles of furniture such as a deacon's bench is what most people strive for. I have even seen situations where people have used small church pews, recycled when a house of worship is torn down.
Typically, the large (comparatively speaking) back porch area is typical of the rural farm house. I have not seen examples of "open lockers" as alluded to by Paul, but I have seen rows of coat hooks or pegs up on one wall where outer garments can be hung. To have one's laundry room off the back porch would be genius where layout is concerned. Actually, some back porches are big enough to locate the washer and dryer directly in them.
My father, when he renovated his house, had a big back entryway with a row of coat pegs up on the wall directly opposite the door; beside this was a French door into the kitchen, and to the left was another door that led into a bathroom (which one could walk through into my parents' bedroom, which also had a door beside this that led from the bedroom back into the kitchen). The beauty of this arrangement was that in the summer, one could come in from doing yardwork outside and take a shower without walking through the whole house.
L1 mentions the "Three Seasons Room", which in most cases is something added to the back of an existing house (although they may be a feature of a newly-built home, also). They are mainly a lot of windows, some of which may be opened to catch summer breezes (they have screen on them to keep insects out), and I have heard of cases where people have actually added electric baseboard heat so that they can also use them in the winter. I'll try to find some pictures of these.