It depends entirely on what you want to do with it.
Some free ones to get you started -
is a very simple tile based mapper. While it will not do miniature scaled maps I find it very useful for quick 'I need something for tonight's game' maps. The link leads to the older versions of the program, the second and third version stalled out before the project was finished. I use a combination of DC 2 and DC1 - making the bulk of the map in one and adding detail with the other. (Details include things like doors - available in DC 1, but not the incomplete DC2) If you look at some of the tiles and objects for the program you may notice some by a fellow with the handle The Auld Grump.
I have a generic adventure or two up there as well.
I use DC primarily for interiors and for village scale maps.
More complex, but much better for large area maps, is a drawing program based mapping application. I have not used it much, preferring commercial products for such endeavors.
For commercial programs I am fond of both which is easy to use, and runs about $35 U.S. and which is $45. Campaign Cartographer 3 is a more involved program, but much, much easier to use than older versions of the program, and more powerful to boot. It is pretty easy to do both overland maps and miniature scaled floorplans for use on the tabletop.
Because of a nifty bit of work by The Fool 1972 (who used to run Ancient Awakening Games) I actually use the older Fractal Mapper 7 more than the current Fractal Mapper 8 - he made some really nice tools for creating town maps.
The Auld Grump