It should be noted that latitudes are a lot less arbitrary than longitude: the latitude of a given point corresponds directly to the elevation angle of the North Star above the horizon at that point; the locus of points at which this elevation is a given number is a circle around the earth.
That's why crossing the 45th parallel is neat- you're halfway between the Equator and the North Pole. Of course, if you started from 42°N, as I did, it's less exciting. However, during the summer, those three degrees of latitude mean noticeably more daylight. This is important for one of two reasons: 1) like a reptile, I need to bask; or 2) like Kal-El, the last son of Krypton, I gain my powers from the Earth's yellow sun. Believe whichever of those you like.
I'm not the only one who thinks this is cool. The park district of Marathon County, Wisconsin, put up a sign on the confluence of 45°N and 90°W. According to the writeup at RoadsideAmerica.com, the very existence of this monument is the culmination of one man's personal quest.
It's worth noting that the marker was probably placed without benefit of GPS. Geographic nerdosity existed long before (artificial) satellites could tell us anything useful. GPS just helped. A lot.
Coordinates: (surprise!) 45°N, 90°W. There's a signed access road from Highway 29.
I visited the site and took this picture over five years ago. The current winter wonderland reminded me of stomping around in the snow to find the marker.