The USGS provides more maps than you can imagine.  Basically, they have printed maps, map CDs, and maps online.  But there are many different types, so I will attempt to explain what they are and where to get them.
       To start with, printed maps are generally $4 each, no matter which type or size they are.  There are a few exceptions, which are free, but not many.  Map CDs, which are available for all states, are $34 each.  However, in some cases it takes more than 1 CD to hold all the data, in which case supplimental CDs in the "set" are an additional $10 each.  Maps online, of course, are free.
        If you need maps for your genealogy work, the first thing you should do is to order the free Index book(s) for the state(s) of interest.  Originally, these came as soft cover books, but have changed recently to sheets (maps).  Each book or sheet shows a state with all the major features on the $4 maps, except that they show a grid of squares representing larger-scale 7½' x 7½' "quadrangles", which have far smaller coverage but far more detail.  If you're on a tight budget, you can use these indexes insted of buying the maps.  If you're not worried about a budget, you should still have these indexes to help you navigate thru the miriad of different maps, scales, types, etc.  The USGS uses a complex system of naming & numbering for their maps, and it helps to have the index, which makes it all clear.
        Here I should point out that while the cost of the CDs sounds very expensive, they're actually a real bargain.  When you see how many "quadrangles" there are in any state, and multiply them by $4, you'll find that it's much cheaper to go with the CDs, unless you want a very few maps (less than 9).
        And, I should also point out before we get into the different map types, that all these printed and CD maps are "modern", in that they show what's there "now".  In my work of platting deeds from the 1600-1800's, I don't want to see all the cities, highways, railroads, and power lines, so I stay away from both the printed and CD maps, and use the "do-it-yourself" maps off the internet.
        The problem with the free maps online is that they aren't really maps -- just mapping data.  By that I mean that you download a file that has all the "water", another for the "state & county boundries", and another for the "elevation contour lines".  By keeping these different files in different "layers" (pages), you can selectivly turn them on and off with a viewer or CAD program to get the map you want.  However, and here's the greatest drawback, THERE IS NO TEXT -- ONLY LINES (see GNIS below).  So, it's a lot of extra work to label all the rivers, streams, towns, etc., but when you're done, you've got a custom map the way you want it.  The only other problem with online maps (mapping data) is that they're only available in certain scales.  The scale that I work with, 1:100,000, covers the area of 4 quadrangles, which means they only have about ¼ the detail available on a quadrangle.  However, with the ability to zoom in on any specific area with my CAD program, and print out just that part, I find it satisfactory for my use.  Now for the different types of maps.  

DLG (Digital Line Graph)
      The type of map (map data) I was just referring to is called a DLG.  DLGs are available in the "optional format" 1:100,000 scale from  USGS 100K DLGs, but you'll need the free index I mentioned to find your way to the file you want.  The downloading and conversion of these files is a bit tricky, so I've made a seperate page telling you how to do it.  Remember, these have NO TEXT.  Also, you can download a very good viewer & print program for these files  DLGV32.exe.
        If you want to "draw" on these with your PC, you'll need a CAD program, and you will have to convert the the files from .dlg to .dxf format.  Luckely, there's a free program that will do this conversion for you  DLGLX133.exe .  
        DLGs are also available in what's called the SDTS format (both quadrangle 1:24,000 and  100K scales), but there are few viewers to work with them that I know of, and the viewer above doesn't.  There's a conversion program to change the .sdts files to .dxf files  SDTS2DXF.exe, but I haven't had much luck with it.  The SDTS DLG files are available at  USGS 24K SDTS DLGs .

DRG (Digital Raster Graphics)
        The CDs I spoke of are DRGs.  They are "picture" copies of all the $4 quadrangle maps, digitized for viewing on PCs.  But, again, they are "modern", and there are no layers to turn on & off.     USGS DRGs

DEM (Digital Elevation Maps)
        These try to give a 3D view of large areas.  While interesting, they aren't of much use to us in genealogy.  There are very few of these online.  If you're curious, you can get a peek by going to  USGS DEMs .

GNIS (Geographical Names Information System)
        GNIS is the place-name database.  You can download a file for any state, but be warned -- they are very large files.  The file for VA is greater than 10.3MB, and for NC is greater than 11.3MB.  These files give the names & Lat/Lon of places, and of beginnings & endings of waterways.  Unfortunately, DLGs, where the names are needed most, use the UTM (Universal Transverse Mercater) system, which is not compatible with Lat/Lon, and one of them (the Lat/Lon) must be converted to merge the two.  Even then, the ends of rivers and streams may lie outside the boundries of the map you're working on, so how do you place the name?  I know of no program that will convert the GNIS Lat/Lon to UTM and place the label on the DLG map.  Unfortunate!  As time permits, I'll try to write a program to do this, but it's probably a long way off (if it's even possible). The GNIS Database.    Make a GNIS Query (ask for a single location).

TIGER
        If all that I've said so far seems too complex for you, I've saved the simplest and easiest for last. The Tiger mapping system is run by the Census Bureau, using USGS data, and will allow you to zoom in or out on any area and print the result.  It doesn't show much detail, and you can't draw on it with your PC, but it's very easy to use.  While it also falls into the "modern" map catagory, you have the ability to turn different "layers" on & off (towns, highways, etc.). 
     If you're trying to find the location of a deed, which uses the Section-Township-&-Range system, try my Twp-Rng Calculator, which will give you the proper map (in a seperate window) & mark the upper-left (NW) corner of the deed with a red stick-pin.

     For additional info on maps, what's available, and ordering, you can check out the USGS home page or contact them at: