Ok, I don't know if the maps are wrong, or the GPS is wrong, but it ALWAYS puts me off the road in a lot of places. I'm driving, and all of a sudden I am 20 meters off a road. And my house, which is on the corner of two streets, is shown way off when I'm standing by it. According to the GPS (or the maps, not too sure), I'm standing in the creek behind my house.

Program: RoadMaps
Maps: OpenStreet (better) and the U.S. Government ones (Shitty).

I've found Google Street very good and haven't had the problem you described.

GPS tends to suffer from reflections, etc., which cause apparent displacements. Mapping apps which assume you're on a road can generally account for thes little hiccups.

Regarding altitude, once you have enough satellites altitude should be reasonably accurate (though not as accurate as the horizontal positioning due to the satellite orbits and the geometry of it all). Though note that the altitude is probably given with respect to a nominal geodetic/ellipsoidal shape rather than the actual sea level.

Which programs, aside from Map, assume you are on the road. Does MaemoMapper do so? RoadMap does not do this, because often times it says I'm driving through buildings and rivers. Its really confusing when you are driving in new place, and all of a sudden the computerized Flite voice says "Approaching Random Street," when you are way far away from it...

Maps - like some consumer GPSes for navigation - assumes you're on a road whilst navigating. When in "normal" mode, it doesn't assume you're on a road AFAICT.

Maemo mapper doesn't assume you're on a road, mainly because it doesn't know where the roads are, it just displays your location on the bitmap it's downloaded.

IIRC NavIt also places you on the nearest road (when using Garmin maps).

Being a GPS noobie I've been having trouble figuring how to create a route with Maemo Mapper -- still no luck. I have however found a web page that creates XML GPX files which Maemo Mapper can open as routes. http://www.gnuite.com/cgi-bin/gpx.cgi

In mm on the menu choose Route > Download (iirc), then stick in the locations and it uses the site you've given the url for to parse the Google maps direction output.

They have to, although that model could be just a simple (and wrong) perfect sphere. It can't give you an altitude above sea level if it doesn't know where the sea level is - it would only know the distance from the center of the Earth.
The Earth looks a bit more pear-shaped than a sphere (wider at the equator and fatter at the south). So, unless you want a wildly incorrect altitude reading you need a model. But a model isn't perfect unless it's big and complicated.

The error is systematic and varies depending on where you are. In my area it seems to be about 38-43 meters (with my GPS. It could be diifferent with other receivers). Walk down to the sea (if possible..) and check out what your GPS says.

(In addition to the systematic error the GPS isn't as good at calculating altitude as longitude/latitude so there's an additional inaccuracy there, but I'm not sure how big it is.)

It depends on where you are. Last time I checked was somewhere above Italy and my GPS was about 100 meters different from the in-flight video.